PowerPC: Fix vDSO missing ODP entries

This patch fixes the vDSO symbol used directed in IFUNC resolver where
they do not have an associated ODP entry leading to undefined behavior
in some cases. It adds an artificial OPD static entry to such cases
and set its TOC to non 0 to avoid triggering lazy resolutions.

3 files changed, 28 insertions, 5 deletions

diff --git a/sysdeps/unix/sysv/linux/powerpc/bits/libc-vdso.h b/sysdeps/unix/sysv/linux/powerpc/bits/libc-vdso.h
index ba54de4eb0..d1891698f5 100644
--- a/sysdeps/unix/sysv/linux/powerpc/bits/libc-vdso.h
+++ b/sysdeps/unix/sysv/linux/powerpc/bits/libc-vdso.h
@@ -41,12 +41,32 @@ extern void *__vdso_sigtramp32;
 extern void *__vdso_sigtramp_rt32;
 #endif
 
-/* This macro is needed for PPC64 to return a skeleton OPD entry of a vDSO
-   symbol.  This works because _dl_vdso_vsym always return the function
-   address, and no vDSO symbols use the TOC or chain pointers from the OPD
-   so we can allow them to be garbage.  */
 #if defined(__PPC64__) || defined(__powerpc64__)
-#define VDSO_IFUNC_RET(value)  ((void *) &(value))
+/* The correct solution is for _dl_vdso_vsym to return the address of the OPD
+   for the kernel VDSO function.  That address would then be stored in the
+   __vdso_* variables and returned as the result of the IFUNC resolver function.
+   Yet, the kernel does not contain any OPD entries for the VDSO functions
+   (incomplete implementation).  However, PLT relocations for IFUNCs still expect
+   the address of an OPD to be returned from the IFUNC resolver function (since
+   PLT entries on PPC64 are just copies of OPDs).  The solution for now is to
+   create an artificial static OPD for each VDSO function returned by a resolver
+   function.  The TOC value is set to a non-zero value to avoid triggering lazy
+   symbol resolution via .glink0/.plt0 for a zero TOC (requires thread-safe PLT
+   sequences) when the dynamic linker isn't prepared for it e.g. RTLD_NOW.  None
+   of the kernel VDSO routines use the TOC or AUX values so any non-zero value
+   will work.  Note that function pointer comparisons will not use this artificial
+   static OPD since those are resolved via ADDR64 relocations and will point at
+   the non-IFUNC default OPD for the symbol.  Lastly, because the IFUNC relocations
+   are processed immediately at startup the resolver functions and this code need
+   not be thread-safe, but if the caller writes to a PLT slot it must do so in a
+   thread-safe manner with all the required barriers.  */
+#define VDSO_IFUNC_RET(value)                            \
+  ({                                                     \
+    static Elf64_FuncDesc vdso_opd = { .fd_toc = ~0x0 }; \
+    vdso_opd.fd_func = (Elf64_Addr)value;                \
+    &vdso_opd;                                           \
+  })
+
 #else
 #define VDSO_IFUNC_RET(value)  ((void *) (value))
 #endif
diff --git a/sysdeps/unix/sysv/linux/powerpc/gettimeofday.c b/sysdeps/unix/sysv/linux/powerpc/gettimeofday.c
index 6506d75e64..48c3f84d86 100644
--- a/sysdeps/unix/sysv/linux/powerpc/gettimeofday.c
+++ b/sysdeps/unix/sysv/linux/powerpc/gettimeofday.c
@@ -22,6 +22,7 @@
 
 # include <dl-vdso.h>
 # include <bits/libc-vdso.h>
+# include <dl-machine.h>
 
 void *gettimeofday_ifunc (void) __asm__ ("__gettimeofday");
 
diff --git a/sysdeps/unix/sysv/linux/powerpc/time.c b/sysdeps/unix/sysv/linux/powerpc/time.c
index 66b4eb3049..2d77ecec9b 100644
--- a/sysdeps/unix/sysv/linux/powerpc/time.c
+++ b/sysdeps/unix/sysv/linux/powerpc/time.c
@@ -20,7 +20,9 @@
 
 # include <time.h>
 # include <sysdep.h>
+# include <dl-vdso.h>
 # include <bits/libc-vdso.h>
+# include <dl-machine.h>
 
 void *time_ifunc (void) asm ("time");