elf: Do not duplicate the GLIBC_TUNABLES string

The tunable parsing duplicates the tunable environment variable so it
null-terminates each one since it simplifies the later parsing. It has
the drawback of adding another point of failure (__minimal_malloc
failing), and the memory copy requires tuning the compiler to avoid mem
operations calls.

The parsing now tracks the tunable start and its size. The
dl-tunable-parse.h adds helper functions to help parsing, like a strcmp
that also checks for size and an iterator for suboptions that are
comma-separated (used on hwcap parsing by x86, powerpc, and s390x).

Since the environment variable is allocated on the stack by the kernel,
it is safe to keep the references to the suboptions for later parsing
of string tunables (as done by set_hwcaps by multiple architectures).

Checked on x86_64-linux-gnu, powerpc64le-linux-gnu, and
aarch64-linux-gnu.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

8 files changed, 424 insertions, 227 deletions

diff --git a/sysdeps/generic/dl-tunables-parse.h b/sysdeps/generic/dl-tunables-parse.h
new file mode 100644
index 0000000000..b37be0443b
--- /dev/null
+++ b/sysdeps/generic/dl-tunables-parse.h
@@ -0,0 +1,134 @@
+/* Helper functions to handle tunable strings.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _DL_TUNABLES_PARSE_H
+#define _DL_TUNABLES_PARSE_H 1
+
+#include <assert.h>
+#include <string.h>
+
+/* Compare the contents of STRVAL with STR of size LEN.  The STR might not
+   be null-terminated.   */
+static __always_inline bool
+tunable_strcmp (const struct tunable_str_t *strval, const char *str,
+		size_t len)
+{
+  return strval->len == len && memcmp (strval->str, str, len) == 0;
+}
+#define tunable_strcmp_cte(__tunable, __str) \
+ tunable_strcmp (&__tunable->strval, __str, sizeof (__str) - 1)
+
+/*
+   Helper functions to iterate over a tunable string composed by multiple
+   suboptions separated by commaxi; this is a common pattern for CPU.  Each
+   suboptions is return in the form of { address, size } (no null terminated).
+   For instance:
+
+     struct tunable_str_comma_t ts;
+     tunable_str_comma_init (&ts, valp);
+
+     struct tunable_str_t t;
+     while (tunable_str_comma_next (&ts, &t))
+      {
+	_dl_printf ("[%s] %.*s (%d)\n",
+		    __func__,
+		    (int) tstr.len,
+		    tstr.str,
+		    (int) tstr.len);
+
+        if (tunable_str_comma_strcmp (&t, opt, opt1_len))
+	  {
+	    [...]
+	  }
+	else if (tunable_str_comma_strcmp_cte (&t, "opt2"))
+	  {
+	    [...]
+	  }
+      }
+
+   NB: These function are expected to be called from tunable callback
+   functions along with tunable_val_t with string types.
+*/
+
+struct tunable_str_comma_state_t
+{
+  const char *p;
+  size_t plen;
+  size_t maxplen;
+};
+
+struct tunable_str_comma_t
+{
+  const char *str;
+  size_t len;
+  bool disable;
+};
+
+static inline void
+tunable_str_comma_init (struct tunable_str_comma_state_t *state,
+			tunable_val_t *valp)
+{
+  assert (valp->strval.str != NULL);
+  state->p = valp->strval.str;
+  state->plen = 0;
+  state->maxplen = valp->strval.len;
+}
+
+static inline bool
+tunable_str_comma_next (struct tunable_str_comma_state_t *state,
+			struct tunable_str_comma_t *str)
+{
+  if (*state->p == '\0' || state->plen >= state->maxplen)
+    return false;
+
+  const char *c;
+  for (c = state->p; *c != ','; c++, state->plen++)
+    if (*c == '\0' || state->plen == state->maxplen)
+      break;
+
+  str->str = state->p;
+  str->len = c - state->p;
+
+  if (str->len > 0)
+    {
+      str->disable = *str->str == '-';
+      if (str->disable)
+	{
+	  str->str = str->str + 1;
+	  str->len = str->len - 1;
+	}
+    }
+
+  state->p = c + 1;
+  state->plen++;
+
+  return true;
+}
+
+/* Compare the contents of T with STR of size LEN.  The STR might not be
+   null-terminated.   */
+static __always_inline bool
+tunable_str_comma_strcmp (const struct tunable_str_comma_t *t, const char *str,
+			  size_t len)
+{
+  return t->len == len && memcmp (t->str, str, len) == 0;
+}
+#define tunable_str_comma_strcmp_cte(__t, __str) \
+  tunable_str_comma_strcmp (__t, __str, sizeof (__str) - 1)
+
+#endif
diff --git a/sysdeps/s390/cpu-features.c b/sysdeps/s390/cpu-features.c
index 55449ba07f..06c1cab0fd 100644
--- a/sysdeps/s390/cpu-features.c
+++ b/sysdeps/s390/cpu-features.c
@@ -22,6 +22,7 @@
 #include <ifunc-memcmp.h>
 #include <string.h>
 #include <dl-symbol-redir-ifunc.h>
+#include <dl-tunables-parse.h>
 
 #define S390_COPY_CPU_FEATURES(SRC_PTR, DEST_PTR)	\
   (DEST_PTR)->hwcap = (SRC_PTR)->hwcap;			\
@@ -51,33 +52,14 @@ TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
   struct cpu_features cpu_features_curr;
   S390_COPY_CPU_FEATURES (cpu_features, &cpu_features_curr);
 
-  const char *token = valp->strval;
-  do
+  struct tunable_str_comma_state_t ts;
+  tunable_str_comma_init (&ts, valp);
+
+  struct tunable_str_comma_t t;
+  while (tunable_str_comma_next (&ts, &t))
     {
-      const char *token_end, *feature;
-      bool disable;
-      size_t token_len;
-      size_t feature_len;
-
-      /* Find token separator or end of string.  */
-      for (token_end = token; *token_end != ','; token_end++)
-	if (*token_end == '\0')
-	  break;
-
-      /* Determine feature.  */
-      token_len = token_end - token;
-      if (*token == '-')
-	{
-	  disable = true;
-	  feature = token + 1;
-	  feature_len = token_len - 1;
-	}
-      else
-	{
-	  disable = false;
-	  feature = token;
-	  feature_len = token_len;
-	}
+      if (t.len == 0)
+	continue;
 
       /* Handle only the features here which are really used in the
 	 IFUNC-resolvers.  All others are ignored as the values are only used
@@ -85,86 +67,64 @@ TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
       bool reset_features = false;
       unsigned long int hwcap_mask = 0UL;
       unsigned long long stfle_bits0_mask = 0ULL;
+      bool disable = t.disable;
 
-      if ((*feature == 'z' || *feature == 'a'))
+      if (tunable_str_comma_strcmp_cte (&t, "zEC12")
+	  || tunable_str_comma_strcmp_cte (&t, "arch10"))
+	{
+	  reset_features = true;
+	  disable = true;
+	  hwcap_mask = HWCAP_S390_VXRS | HWCAP_S390_VXRS_EXT
+	    | HWCAP_S390_VXRS_EXT2;
+	  stfle_bits0_mask = S390_STFLE_MASK_ARCH13_MIE3;
+	}
+      else if (tunable_str_comma_strcmp_cte (&t, "z13")
+	       || tunable_str_comma_strcmp_cte (&t, "arch11"))
+	{
+	  reset_features = true;
+	  disable = true;
+	  hwcap_mask = HWCAP_S390_VXRS_EXT | HWCAP_S390_VXRS_EXT2;
+	  stfle_bits0_mask = S390_STFLE_MASK_ARCH13_MIE3;
+	}
+      else if (tunable_str_comma_strcmp_cte (&t, "z14")
+	       || tunable_str_comma_strcmp_cte (&t, "arch12"))
+	{
+	  reset_features = true;
+	  disable = true;
+	  hwcap_mask = HWCAP_S390_VXRS_EXT2;
+	  stfle_bits0_mask = S390_STFLE_MASK_ARCH13_MIE3;
+	}
+      else if (tunable_str_comma_strcmp_cte (&t, "z15")
+	       || tunable_str_comma_strcmp_cte (&t, "z16")
+	       || tunable_str_comma_strcmp_cte (&t, "arch13")
+	       || tunable_str_comma_strcmp_cte (&t, "arch14"))
 	{
-	  if ((feature_len == 5 && *feature == 'z'
-	       && memcmp (feature, "zEC12", 5) == 0)
-	      || (feature_len == 6 && *feature == 'a'
-		  && memcmp (feature, "arch10", 6) == 0))
-	    {
-	      reset_features = true;
-	      disable = true;
-	      hwcap_mask = HWCAP_S390_VXRS | HWCAP_S390_VXRS_EXT
-		| HWCAP_S390_VXRS_EXT2;
-	      stfle_bits0_mask = S390_STFLE_MASK_ARCH13_MIE3;
-	    }
-	  else if ((feature_len == 3 && *feature == 'z'
-		    && memcmp (feature, "z13", 3) == 0)
-		   || (feature_len == 6 && *feature == 'a'
-		       && memcmp (feature, "arch11", 6) == 0))
-	    {
-	      reset_features = true;
-	      disable = true;
-	      hwcap_mask = HWCAP_S390_VXRS_EXT | HWCAP_S390_VXRS_EXT2;
-	      stfle_bits0_mask = S390_STFLE_MASK_ARCH13_MIE3;
-	    }
-	  else if ((feature_len == 3 && *feature == 'z'
-		    && memcmp (feature, "z14", 3) == 0)
-		   || (feature_len == 6 && *feature == 'a'
-		       && memcmp (feature, "arch12", 6) == 0))
-	    {
-	      reset_features = true;
-	      disable = true;
-	      hwcap_mask = HWCAP_S390_VXRS_EXT2;
-	      stfle_bits0_mask = S390_STFLE_MASK_ARCH13_MIE3;
-	    }
-	  else if ((feature_len == 3 && *feature == 'z'
-		    && (memcmp (feature, "z15", 3) == 0
-			|| memcmp (feature, "z16", 3) == 0))
-		   || (feature_len == 6
-		       && (memcmp (feature, "arch13", 6) == 0
-			   || memcmp (feature, "arch14", 6) == 0)))
-	    {
-	      /* For z15 or newer we don't have to disable something,
-		 but we have to reset to the original values.  */
-	      reset_features = true;
-	    }
+	  /* For z15 or newer we don't have to disable something, but we have
+	     to reset to the original values.  */
+	  reset_features = true;
 	}
-      else if (*feature == 'H')
+      else if (tunable_str_comma_strcmp_cte (&t, "HWCAP_S390_VXRS"))
 	{
-	  if (feature_len == 15
-	      && memcmp (feature, "HWCAP_S390_VXRS", 15) == 0)
-	    {
-	      hwcap_mask = HWCAP_S390_VXRS;
-	      if (disable)
-		hwcap_mask |= HWCAP_S390_VXRS_EXT | HWCAP_S390_VXRS_EXT2;
-	    }
-	  else if (feature_len == 19
-		   && memcmp (feature, "HWCAP_S390_VXRS_EXT", 19) == 0)
-	    {
-	      hwcap_mask = HWCAP_S390_VXRS_EXT;
-	      if (disable)
-		hwcap_mask |= HWCAP_S390_VXRS_EXT2;
-	      else
-		hwcap_mask |= HWCAP_S390_VXRS;
-	    }
-	  else if (feature_len == 20
-		   && memcmp (feature, "HWCAP_S390_VXRS_EXT2", 20) == 0)
-	    {
-	      hwcap_mask = HWCAP_S390_VXRS_EXT2;
-	      if (!disable)
-		hwcap_mask |= HWCAP_S390_VXRS | HWCAP_S390_VXRS_EXT;
-	    }
+	  hwcap_mask = HWCAP_S390_VXRS;
+	  if (t.disable)
+	    hwcap_mask |= HWCAP_S390_VXRS_EXT | HWCAP_S390_VXRS_EXT2;
 	}
-      else if (*feature == 'S')
+      else if (tunable_str_comma_strcmp_cte (&t, "HWCAP_S390_VXRS_EXT"))
 	{
-	  if (feature_len == 10
-	      && memcmp (feature, "STFLE_MIE3", 10) == 0)
-	    {
-	      stfle_bits0_mask = S390_STFLE_MASK_ARCH13_MIE3;
-	    }
+	  hwcap_mask = HWCAP_S390_VXRS_EXT;
+	  if (t.disable)
+	    hwcap_mask |= HWCAP_S390_VXRS_EXT2;
+	  else
+	    hwcap_mask |= HWCAP_S390_VXRS;
+	}
+      else if (tunable_str_comma_strcmp_cte (&t, "HWCAP_S390_VXRS_EXT2"))
+	{
+	  hwcap_mask = HWCAP_S390_VXRS_EXT2;
+	  if (!t.disable)
+	    hwcap_mask |= HWCAP_S390_VXRS | HWCAP_S390_VXRS_EXT;
 	}
+      else if (tunable_str_comma_strcmp_cte (&t, "STFLE_MIE3"))
+	stfle_bits0_mask = S390_STFLE_MASK_ARCH13_MIE3;
 
       /* Perform the actions determined above.  */
       if (reset_features)
@@ -187,14 +147,7 @@ TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
 	  else
 	    cpu_features_curr.stfle_bits[0] |= stfle_bits0_mask;
 	}
-
-      /* Jump over current token ... */
-      token += token_len;
-
-      /* ... and skip token separator for next round.  */
-      if (*token == ',') token++;
     }
-  while (*token != '\0');
 
   /* Copy back the features after checking that no unsupported features were
      enabled by user.  */
diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
index a11a86efab..c57f154b48 100644
--- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
+++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
@@ -16,10 +16,12 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <array_length.h>
 #include <cpu-features.h>
 #include <sys/auxv.h>
 #include <elf/dl-hwcaps.h>
 #include <sys/prctl.h>
+#include <dl-tunables-parse.h>
 
 #define DCZID_DZP_MASK (1 << 4)
 #define DCZID_BS_MASK (0xf)
@@ -33,25 +35,28 @@
 struct cpu_list
 {
   const char *name;
+  size_t len;
   uint64_t midr;
 };
 
-static struct cpu_list cpu_list[] = {
-      {"thunderxt88",	 0x430F0A10},
-      {"thunderx2t99",   0x431F0AF0},
-      {"thunderx2t99p1", 0x420F5160},
-      {"ares",		 0x411FD0C0},
-      {"emag",		 0x503F0001},
-      {"kunpeng920", 	 0x481FD010},
-      {"a64fx",		 0x460F0010},
-      {"generic", 	 0x0}
+static const struct cpu_list cpu_list[] =
+{
+#define CPU_LIST_ENTRY(__str, __num) { __str, sizeof (__str) - 1, __num }
+  CPU_LIST_ENTRY ("thunderxt88",    0x430F0A10),
+  CPU_LIST_ENTRY ("thunderx2t99",   0x431F0AF0),
+  CPU_LIST_ENTRY ("thunderx2t99p1", 0x420F5160),
+  CPU_LIST_ENTRY ("ares",           0x411FD0C0),
+  CPU_LIST_ENTRY ("emag",           0x503F0001),
+  CPU_LIST_ENTRY ("kunpeng920",     0x481FD010),
+  CPU_LIST_ENTRY ("a64fx",          0x460F0010),
+  CPU_LIST_ENTRY ("generic",        0x0),
 };
 
 static uint64_t
-get_midr_from_mcpu (const char *mcpu)
+get_midr_from_mcpu (const struct tunable_str_t *mcpu)
 {
-  for (int i = 0; i < sizeof (cpu_list) / sizeof (struct cpu_list); i++)
-    if (strcmp (mcpu, cpu_list[i].name) == 0)
+  for (int i = 0; i < array_length (cpu_list); i++)
+    if (tunable_strcmp (mcpu, cpu_list[i].name, cpu_list[i].len))
       return cpu_list[i].midr;
 
   return UINT64_MAX;
@@ -63,7 +68,9 @@ init_cpu_features (struct cpu_features *cpu_features)
   register uint64_t midr = UINT64_MAX;
 
   /* Get the tunable override.  */
-  const char *mcpu = TUNABLE_GET (glibc, cpu, name, const char *, NULL);
+  const struct tunable_str_t *mcpu = TUNABLE_GET (glibc, cpu, name,
+						  struct tunable_str_t *,
+						  NULL);
   if (mcpu != NULL)
     midr = get_midr_from_mcpu (mcpu);
 
diff --git a/sysdeps/unix/sysv/linux/powerpc/cpu-features.c b/sysdeps/unix/sysv/linux/powerpc/cpu-features.c
index 7c6e20e702..390b3fd11a 100644
--- a/sysdeps/unix/sysv/linux/powerpc/cpu-features.c
+++ b/sysdeps/unix/sysv/linux/powerpc/cpu-features.c
@@ -20,6 +20,7 @@
 #include <stdint.h>
 #include <cpu-features.h>
 #include <elf/dl-tunables.h>
+#include <dl-tunables-parse.h>
 #include <unistd.h>
 #include <string.h>
 
@@ -43,41 +44,26 @@ TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
   struct cpu_features *cpu_features = &GLRO(dl_powerpc_cpu_features);
   unsigned long int tcbv_hwcap = cpu_features->hwcap;
   unsigned long int tcbv_hwcap2 = cpu_features->hwcap2;
-  const char *token = valp->strval;
-  do
+
+  struct tunable_str_comma_state_t ts;
+  tunable_str_comma_init (&ts, valp);
+
+  struct tunable_str_comma_t t;
+  while (tunable_str_comma_next (&ts, &t))
     {
-      const char *token_end, *feature;
-      bool disable;
-      size_t token_len, i, feature_len, offset = 0;
-      /* Find token separator or end of string.  */
-      for (token_end = token; *token_end != ','; token_end++)
-	if (*token_end == '\0')
-	  break;
+      if (t.len == 0)
+	continue;
 
-      /* Determine feature.  */
-      token_len = token_end - token;
-      if (*token == '-')
-	{
-	  disable = true;
-	  feature = token + 1;
-	  feature_len = token_len - 1;
-	}
-      else
-	{
-	  disable = false;
-	  feature = token;
-	  feature_len = token_len;
-	}
-      for (i = 0; i < array_length (hwcap_tunables); ++i)
+      size_t offset = 0;
+      for (int i = 0; i < array_length (hwcap_tunables); ++i)
 	{
 	  const char *hwcap_name = hwcap_names + offset;
 	  size_t hwcap_name_len = strlen (hwcap_name);
 	  /* Check the tunable name on the supported list.  */
-	  if (hwcap_name_len == feature_len
-	      && memcmp (feature, hwcap_name, feature_len) == 0)
+	  if (tunable_str_comma_strcmp (&t, hwcap_name, hwcap_name_len))
 	    {
 	      /* Update the hwcap and hwcap2 bits.  */
-	      if (disable)
+	      if (t.disable)
 		{
 		  /* Id is 1 for hwcap2 tunable.  */
 		  if (hwcap_tunables[i].id)
@@ -98,12 +84,7 @@ TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
 	    }
 	  offset += hwcap_name_len + 1;
 	}
-	token += token_len;
-	/* ... and skip token separator for next round.  */
-	if (*token == ',')
-	  token++;
     }
-  while (*token != '\0');
 }
 
 static inline void
diff --git a/sysdeps/unix/sysv/linux/powerpc/tst-hwcap-tunables.c b/sysdeps/unix/sysv/linux/powerpc/tst-hwcap-tunables.c
index 2631016a3a..049164f841 100644
--- a/sysdeps/unix/sysv/linux/powerpc/tst-hwcap-tunables.c
+++ b/sysdeps/unix/sysv/linux/powerpc/tst-hwcap-tunables.c
@@ -110,7 +110,11 @@ do_test (int argc, char *argv[])
 	run_test ("-arch_2_06", "__memcpy_power7");
       if (hwcap & PPC_FEATURE_ARCH_2_05)
 	run_test ("-arch_2_06,-arch_2_05","__memcpy_power6");
-      run_test ("-arch_2_06,-arch_2_05,-power5+,-power5,-power4", "__memcpy_power4");
+      run_test ("-arch_2_06,-arch_2_05,-power5+,-power5,-power4",
+		"__memcpy_power4");
+      /* Also run with valid, but empty settings.  */
+      run_test (",-,-arch_2_06,-arch_2_05,-power5+,-power5,,-power4,-",
+		"__memcpy_power4");
     }
   else
     {
diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile
index 6911a07a87..0aafd2afeb 100644
--- a/sysdeps/x86/Makefile
+++ b/sysdeps/x86/Makefile
@@ -17,6 +17,7 @@ tests += \
   tst-cpu-features-cpuinfo-static \
   tst-cpu-features-supports \
   tst-cpu-features-supports-static \
+  tst-hwcap-tunables \
 # tests
 tests-static += \
   tst-get-cpu-features-static \
@@ -80,6 +81,7 @@ $(objpfx)tst-isa-level-1.out: $(objpfx)tst-isa-level-mod-1-baseline.so \
 endif
 tst-ifunc-isa-2-ENV = GLIBC_TUNABLES=glibc.cpu.hwcaps=-SSE4_2,-AVX,-AVX2,-AVX512F
 tst-ifunc-isa-2-static-ENV = $(tst-ifunc-isa-2-ENV)
+tst-hwcap-tunables-ARGS = -- $(host-test-program-cmd)
 endif
 
 ifeq ($(subdir),math)
diff --git a/sysdeps/x86/cpu-tunables.c b/sysdeps/x86/cpu-tunables.c
index 5697885226..ef96148d30 100644
--- a/sysdeps/x86/cpu-tunables.c
+++ b/sysdeps/x86/cpu-tunables.c
@@ -24,11 +24,12 @@
 #include <string.h>
 #include <cpu-features.h>
 #include <ldsodefs.h>
+#include <dl-tunables-parse.h>
 #include <dl-symbol-redir-ifunc.h>
 
 #define CHECK_GLIBC_IFUNC_CPU_OFF(f, cpu_features, name, len)		\
   _Static_assert (sizeof (#name) - 1 == len, #name " != " #len);	\
-  if (memcmp (f, #name, len) == 0)					\
+  if (tunable_str_comma_strcmp_cte (&f, #name))				\
     {									\
       CPU_FEATURE_UNSET (cpu_features, name)				\
       break;								\
@@ -38,7 +39,7 @@
    which isn't available.  */
 #define CHECK_GLIBC_IFUNC_PREFERRED_OFF(f, cpu_features, name, len)	\
   _Static_assert (sizeof (#name) - 1 == len, #name " != " #len);	\
-  if (memcmp (f, #name, len) == 0)					\
+  if (tunable_str_comma_strcmp_cte (&f, #name) == 0)			\
     {									\
       cpu_features->preferred[index_arch_##name]			\
 	&= ~bit_arch_##name;						\
@@ -46,12 +47,11 @@
     }
 
 /* Enable/disable a preferred feature NAME.  */
-#define CHECK_GLIBC_IFUNC_PREFERRED_BOTH(f, cpu_features, name,	\
-					  disable, len)			\
+#define CHECK_GLIBC_IFUNC_PREFERRED_BOTH(f, cpu_features, name, len)	\
   _Static_assert (sizeof (#name) - 1 == len, #name " != " #len);	\
-  if (memcmp (f, #name, len) == 0)					\
+  if (tunable_str_comma_strcmp_cte (&f, #name))				\
     {									\
-      if (disable)							\
+      if (f.disable)							\
 	cpu_features->preferred[index_arch_##name] &= ~bit_arch_##name;	\
       else								\
 	cpu_features->preferred[index_arch_##name] |= bit_arch_##name;	\
@@ -61,11 +61,11 @@
 /* Enable/disable a preferred feature NAME.  Enable a preferred feature
    only if the feature NEED is usable.  */
 #define CHECK_GLIBC_IFUNC_PREFERRED_NEED_BOTH(f, cpu_features, name,	\
-					       need, disable, len)	\
+					      need, len)		\
   _Static_assert (sizeof (#name) - 1 == len, #name " != " #len);	\
-  if (memcmp (f, #name, len) == 0)					\
+  if (tunable_str_comma_strcmp_cte (&f, #name))				\
     {									\
-      if (disable)							\
+      if (f.disable)							\
 	cpu_features->preferred[index_arch_##name] &= ~bit_arch_##name;	\
       else if (CPU_FEATURE_USABLE_P (cpu_features, need))		\
 	cpu_features->preferred[index_arch_##name] |= bit_arch_##name;	\
@@ -93,38 +93,20 @@ TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
      NOTE: the IFUNC selection may change over time.  Please check all
      multiarch implementations when experimenting.  */
 
-  const char *p = valp->strval, *c;
   struct cpu_features *cpu_features = &GLRO(dl_x86_cpu_features);
-  size_t len;
 
-  do
-    {
-      const char *n;
-      bool disable;
-      size_t nl;
-
-      for (c = p; *c != ','; c++)
-	if (*c == '\0')
-	  break;
+  struct tunable_str_comma_state_t ts;
+  tunable_str_comma_init (&ts, valp);
 
-      len = c - p;
-      disable = *p == '-';
-      if (disable)
-	{
-	  n = p + 1;
-	  nl = len - 1;
-	}
-      else
-	{
-	  n = p;
-	  nl = len;
-	}
-      switch (nl)
+  struct tunable_str_comma_t n;
+  while (tunable_str_comma_next (&ts, &n))
+    {
+      switch (n.len)
 	{
 	default:
 	  break;
 	case 3:
-	  if (disable)
+	  if (n.disable)
 	    {
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, AVX, 3);
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, CX8, 3);
@@ -135,7 +117,7 @@ TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
 	    }
 	  break;
 	case 4:
-	  if (disable)
+	  if (n.disable)
 	    {
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, AVX2, 4);
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, BMI1, 4);
@@ -149,7 +131,7 @@ TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
 	    }
 	  break;
 	case 5:
-	  if (disable)
+	  if (n.disable)
 	    {
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, LZCNT, 5);
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, MOVBE, 5);
@@ -159,12 +141,12 @@ TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
 	    }
 	  break;
 	case 6:
-	  if (disable)
+	  if (n.disable)
 	    {
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, POPCNT, 6);
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, SSE4_1, 6);
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, SSE4_2, 6);
-	      if (memcmp (n, "XSAVEC", 6) == 0)
+	      if (memcmp (n.str, "XSAVEC", 6) == 0)
 		{
 		  /* Update xsave_state_size to XSAVE state size.  */
 		  cpu_features->xsave_state_size
@@ -174,14 +156,14 @@ TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
 	    }
 	  break;
 	case 7:
-	  if (disable)
+	  if (n.disable)
 	    {
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, AVX512F, 7);
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, OSXSAVE, 7);
 	    }
 	  break;
 	case 8:
-	  if (disable)
+	  if (n.disable)
 	    {
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, AVX512CD, 8);
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, AVX512BW, 8);
@@ -190,86 +172,72 @@ TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, AVX512PF, 8);
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, AVX512VL, 8);
 	    }
-	  CHECK_GLIBC_IFUNC_PREFERRED_BOTH (n, cpu_features, Slow_BSF,
-					    disable, 8);
+	  CHECK_GLIBC_IFUNC_PREFERRED_BOTH (n, cpu_features, Slow_BSF, 8);
 	  break;
 	case 11:
 	    {
-	      CHECK_GLIBC_IFUNC_PREFERRED_BOTH (n, cpu_features,
-						Prefer_ERMS,
-						disable, 11);
-	      CHECK_GLIBC_IFUNC_PREFERRED_BOTH (n, cpu_features,
-						Prefer_FSRM,
-						disable, 11);
+	      CHECK_GLIBC_IFUNC_PREFERRED_BOTH (n, cpu_features, Prefer_ERMS,
+						11);
+	      CHECK_GLIBC_IFUNC_PREFERRED_BOTH (n, cpu_features, Prefer_FSRM,
+						11);
 	      CHECK_GLIBC_IFUNC_PREFERRED_NEED_BOTH (n, cpu_features,
 						     Slow_SSE4_2,
 						     SSE4_2,
-						     disable, 11);
+						     11);
 	    }
 	  break;
 	case 15:
 	    {
 	      CHECK_GLIBC_IFUNC_PREFERRED_BOTH (n, cpu_features,
-						Fast_Rep_String,
-						disable, 15);
+						Fast_Rep_String, 15);
 	    }
 	  break;
 	case 16:
 	    {
 	      CHECK_GLIBC_IFUNC_PREFERRED_NEED_BOTH
-		(n, cpu_features, Prefer_No_AVX512, AVX512F,
-		 disable, 16);
+		(n, cpu_features, Prefer_No_AVX512, AVX512F, 16);
 	    }
 	  break;
 	case 18:
 	    {
 	      CHECK_GLIBC_IFUNC_PREFERRED_BOTH (n, cpu_features,
-						Fast_Copy_Backward,
-						disable, 18);
+						Fast_Copy_Backward, 18);
 	    }
 	  break;
 	case 19:
 	    {
 	      CHECK_GLIBC_IFUNC_PREFERRED_BOTH (n, cpu_features,
-						Fast_Unaligned_Load,
-						disable, 19);
+						Fast_Unaligned_Load, 19);
 	      CHECK_GLIBC_IFUNC_PREFERRED_BOTH (n, cpu_features,
-						Fast_Unaligned_Copy,
-						disable, 19);
+						Fast_Unaligned_Copy, 19);
 	    }
 	  break;
 	case 20:
 	    {
 	      CHECK_GLIBC_IFUNC_PREFERRED_NEED_BOTH
-		(n, cpu_features, Prefer_No_VZEROUPPER, AVX, disable,
-		 20);
+		(n, cpu_features, Prefer_No_VZEROUPPER, AVX, 20);
 	    }
 	  break;
 	case 23:
 	    {
 	      CHECK_GLIBC_IFUNC_PREFERRED_NEED_BOTH
-		(n, cpu_features, AVX_Fast_Unaligned_Load, AVX,
-		 disable, 23);
+		(n, cpu_features, AVX_Fast_Unaligned_Load, AVX, 23);
 	    }
 	  break;
 	case 24:
 	    {
 	      CHECK_GLIBC_IFUNC_PREFERRED_NEED_BOTH
-		(n, cpu_features, MathVec_Prefer_No_AVX512, AVX512F,
-		 disable, 24);
+		(n, cpu_features, MathVec_Prefer_No_AVX512, AVX512F, 24);
 	    }
 	  break;
 	case 26:
 	    {
 	      CHECK_GLIBC_IFUNC_PREFERRED_NEED_BOTH
-		(n, cpu_features, Prefer_PMINUB_for_stringop, SSE2,
-		 disable, 26);
+		(n, cpu_features, Prefer_PMINUB_for_stringop, SSE2, 26);
 	    }
 	  break;
 	}
-      p += len + 1;
     }
-  while (*c != '\0');
 }
 
 #if CET_ENABLED
@@ -277,11 +245,11 @@ attribute_hidden
 void
 TUNABLE_CALLBACK (set_x86_ibt) (tunable_val_t *valp)
 {
-  if (memcmp (valp->strval, "on", sizeof ("on")) == 0)
+  if (tunable_strcmp_cte (valp, "on"))
     GL(dl_x86_feature_control).ibt = cet_always_on;
-  else if (memcmp (valp->strval, "off", sizeof ("off")) == 0)
+  else if (tunable_strcmp_cte (valp, "off"))
     GL(dl_x86_feature_control).ibt = cet_always_off;
-  else if (memcmp (valp->strval, "permissive", sizeof ("permissive")) == 0)
+  else if (tunable_strcmp_cte (valp, "permissive"))
     GL(dl_x86_feature_control).ibt = cet_permissive;
 }
 
@@ -289,11 +257,11 @@ attribute_hidden
 void
 TUNABLE_CALLBACK (set_x86_shstk) (tunable_val_t *valp)
 {
-  if (memcmp (valp->strval, "on", sizeof ("on")) == 0)
+  if (tunable_strcmp_cte (valp, "on"))
     GL(dl_x86_feature_control).shstk = cet_always_on;
-  else if (memcmp (valp->strval, "off", sizeof ("off")) == 0)
+  else if (tunable_strcmp_cte (valp, "off"))
     GL(dl_x86_feature_control).shstk = cet_always_off;
-  else if (memcmp (valp->strval, "permissive", sizeof ("permissive")) == 0)
+  else if (tunable_strcmp_cte (valp, "permissive"))
     GL(dl_x86_feature_control).shstk = cet_permissive;
 }
 #endif
diff --git a/sysdeps/x86/tst-hwcap-tunables.c b/sysdeps/x86/tst-hwcap-tunables.c
new file mode 100644
index 0000000000..01a9377f7e
--- /dev/null
+++ b/sysdeps/x86/tst-hwcap-tunables.c
@@ -0,0 +1,148 @@
+/* Tests for x86 GLIBC_TUNABLES=glibc.cpu.hwcaps filter.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <array_length.h>
+#include <getopt.h>
+#include <ifunc-impl-list.h>
+#include <spawn.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <intprops.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/xunistd.h>
+#include <support/capture_subprocess.h>
+
+/* Nonzero if the program gets called via `exec'.  */
+#define CMDLINE_OPTIONS \
+  { "restart", no_argument, &restart, 1 },
+static int restart;
+
+/* Disable everything.  */
+static const char *test_1[] =
+{
+  "__memcpy_avx512_no_vzeroupper",
+  "__memcpy_avx512_unaligned",
+  "__memcpy_avx512_unaligned_erms",
+  "__memcpy_evex_unaligned",
+  "__memcpy_evex_unaligned_erms",
+  "__memcpy_avx_unaligned",
+  "__memcpy_avx_unaligned_erms",
+  "__memcpy_avx_unaligned_rtm",
+  "__memcpy_avx_unaligned_erms_rtm",
+  "__memcpy_ssse3",
+};
+
+static const struct test_t
+{
+  const char *env;
+  const char *const *funcs;
+  size_t nfuncs;
+} tests[] =
+{
+  {
+    /* Disable everything.  */
+    "-Prefer_ERMS,-Prefer_FSRM,-AVX,-AVX2,-AVX_Usable,-AVX2_Usable,"
+    "-AVX512F_Usable,-SSE4_1,-SSE4_2,-SSSE3,-Fast_Unaligned_Load,-ERMS,"
+    "-AVX_Fast_Unaligned_Load",
+    test_1,
+    array_length (test_1)
+  },
+  {
+    /* Same as before, but with some empty suboptions.  */
+    ",-,-Prefer_ERMS,-Prefer_FSRM,-AVX,-AVX2,-AVX_Usable,-AVX2_Usable,"
+    "-AVX512F_Usable,-SSE4_1,-SSE4_2,,-SSSE3,-Fast_Unaligned_Load,,-,-ERMS,"
+    "-AVX_Fast_Unaligned_Load,-,",
+    test_1,
+    array_length (test_1)
+  }
+};
+
+/* Called on process re-execution.  */
+_Noreturn static void
+handle_restart (int ntest)
+{
+  struct libc_ifunc_impl impls[32];
+  int cnt = __libc_ifunc_impl_list ("memcpy", impls, array_length (impls));
+  if (cnt == 0)
+    _exit (EXIT_SUCCESS);
+  TEST_VERIFY_EXIT (cnt >= 1);
+  for (int i = 0; i < cnt; i++)
+    {
+      for (int f = 0; f < tests[ntest].nfuncs; f++)
+	{
+	  if (strcmp (impls[i].name, tests[ntest].funcs[f]) == 0)
+	    TEST_COMPARE (impls[i].usable, false);
+	}
+    }
+
+  _exit (EXIT_SUCCESS);
+}
+
+static int
+do_test (int argc, char *argv[])
+{
+  /* We must have either:
+     - One our fource parameters left if called initially:
+       + path to ld.so         optional
+       + "--library-path"      optional
+       + the library path      optional
+       + the application name
+       + the test to check  */
+
+  TEST_VERIFY_EXIT (argc == 2 || argc == 5);
+
+  if (restart)
+    handle_restart (atoi (argv[1]));
+
+  char nteststr[INT_BUFSIZE_BOUND (int)];
+
+  char *spargv[10];
+  {
+    int i = 0;
+    for (; i < argc - 1; i++)
+      spargv[i] = argv[i + 1];
+    spargv[i++] = (char *) "--direct";
+    spargv[i++] = (char *) "--restart";
+    spargv[i++] = nteststr;
+    spargv[i] = NULL;
+  }
+
+  for (int i = 0; i < array_length (tests); i++)
+    {
+      snprintf (nteststr, sizeof nteststr, "%d", i);
+
+      printf ("[%d] Spawned test for %s\n", i, tests[i].env);
+      char *tunable = xasprintf ("glibc.cpu.hwcaps=%s", tests[i].env);
+      setenv ("GLIBC_TUNABLES", tunable, 1);
+
+      struct support_capture_subprocess result
+	= support_capture_subprogram (spargv[0], spargv);
+      support_capture_subprocess_check (&result, "tst-tunables", 0,
+					sc_allow_stderr);
+      support_capture_subprocess_free (&result);
+
+      free (tunable);
+    }
+
+  return 0;
+}
+
+#define TEST_FUNCTION_ARGV do_test
+#include <support/test-driver.c>