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commit 4486c579cbf0d989080705f515d08cb48636ba88 disabled vdso
clock_gettime on arm due to a Linux kernel bug that was not understood
at the time, whereby the vdso function silently produced
catastrophically wrong results on some systems.
since then, the bug was tracked down to the way the arm kernel
disabled use of vdso clock_gettime on kernels where the necessary
timer was not available or was disabled. it simply patched out the
symbols, but it only did this for the legacy time32 functions, and
left the time64 function in place but non-operational. kernel commit
4405bdf3c57ec28d606bdf5325f1167505bfdcd4 (first present in 5.8)
provided the fix.
if this were a bug that impacted all users of the broken kernel
versions, we could probably ignore it and assume it had been patched
or replaced. however, it's very possible that these kernels appear in
the wild in devices running time32 userspace (glibc, musl 1.1.x, or
some other environment) where they appear to work fine, but where our
new binaries would fail catastrophically if we used the time64 vdso
function.
since the kernel has not (yet?) given us a way to probe for the
working time64 vdso function semantically, we work around the problem
by refusing to use the time64 one unless the time32 one is also
present. this will revert to not using vdso at all if the time32 one
is ever removed, but at least that's safe against wrong results and is
just a missed optimization.
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it's been reported that the vdso clock_gettime64 function on (32-bit)
arm is broken, producing erratic results that grow at a rate far
greater than one reported second per actual elapsed second. the vdso
function seems to have been added sometime between linux 5.4 and 5.6,
so if there's ever been a working version, it was only present for a
very short window.
it's not clear what the eventual upstream kernel solution will be, but
something needs to be done on the libc side so as not to be producing
binaries that seem to work on older/existing/lts kernels (which lack
the function and thus lack the bug) but will break fantastically when
moving to newer kernels.
hopefully vdso support will be added back soon, but with a new symbol
name or version from the kernel to allow continued rejection of broken
ones.
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this commit preserves ABI fully for existing interface boundaries
between libc and libc consumers (applications or libraries), by
retaining existing symbol names for the legacy 32-bit interfaces and
redirecting sources compiled against the new headers to alternate
symbol names. this does not necessarily, however, preserve the
pairwise ABI of libc consumers with one another; where they use
time_t-derived types in their interfaces with one another, it may be
necessary to synchronize updates with each other.
the intent is that ABI resulting from this commit already be stable
and permanent, but it will not be officially so until a release is
made. changes to some header-defined types that do not play any role
in the ABI between libc and its consumers may still be subject to
change.
mechanically, the changes made by this commit for each 32-bit arch are
as follows:
- _REDIR_TIME64 is defined to activate the symbol redirections in
public headers
- COMPAT_SRC_DIRS is defined in arch.mak to activate build of ABI
compat shims to serve as definitions for the original symbol names
- time_t and suseconds_t definitions are changed to long long (64-bit)
- IPC_STAT definition is changed to add the IPC_TIME64 bit (0x100),
triggering conversion of semid_ds, shmid_ds, and msqid_ds split
low/high time bits into new time_t members
- structs semid_ds, shmid_ds, msqid_ds, and stat are modified to add
new 64-bit time_t/timespec members at the end, maintaining existing
layout of other members.
- socket options (SO_*) and ioctl (sockios) command macros are
redefined to use the kernel's "_NEW" values.
in addition, on archs where vdso clock_gettime is used, the
VDSO_CGT_SYM macro definition in syscall_arch.h is changed to use a
new time64 vdso function if available, and a new VDSO_CGT32_SYM macro
is added for use as fallback on kernels lacking time64.
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the mode member of struct ipc_perm is specified by POSIX to have type
mode_t, which is uniformly defined as unsigned int. however, Linux
defines it with type __kernel_mode_t, and defines __kernel_mode_t as
unsigned short on some archs. since there is a subsequent padding
field, treating it as a 32-bit unsigned int works on little endian
archs, but the order is backwards on big endian archs with the
erroneous definition.
since multiple archs are affected, remedy the situation with fixup
code in the affected functions (shmctl, semctl, and msgctl) rather
than repeating the same shims in syscall_arch.h for every affected
arch.
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in thumb mode, r7 is the ABI frame pointer register, and unless frame
pointer is disabled, gcc insists on treating it as a fixed register,
refusing to spill it to satisfy constraints. unfortunately, r7 is also
used in the syscall ABI for passing the syscall number.
up til now we just treated this as a requirement to disable frame
pointer when generating code as thumb, but it turns out gcc forcibly
enables frame pointer, and the fixed register constraint that goes
with it, for functions which contain VLAs. this produces an
unacceptable arch-specific constraint that (non-arm-specific) source
files making syscalls cannot use VLAs.
as a workaround, avoid r7 register constraints when producing thumb
code and instead save/restore r7 in a temp register as part of the asm
block. at some point we may want/need to support armv6-m/thumb1, so
the asm has been tweaked to be thumb1-compatible while also
near-optimal for thumb2: it allows the temp and/or syscall number to
be in high registers (necessary since r0-r5 may all be used for
syscalll args) and in thumb2 mode allows the syscall number to be an
8-bit immediate.
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commit 6d38c9cf80f47623e5e48190046673bbd0dc410b provided an
arm-specific version of posix_fadvise to address the alternate
argument order the kernel expects on arm, but neglected to address
that powerpc (32-bit) has the same issue. instead of having arch
variant files in duplicate, simply put the alternate version in the
top-level file under the control of a macro defined in syscall_arch.h.
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vdso will be available on arm in linux v4.2, the user-space code
for it is in kernel commit 8512287a8165592466cb9cb347ba94892e9c56a5
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remove __syscall declaration where it is not needed (aarch64, arm,
microblaze, or1k) and add the hidden attribute where it is (mips).
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the register constraints in the non-clang case were tested to work on
clang back to 3.2, and earlier versions of clang have known bugs that
preclude building musl.
there may be other reasons to prefer not to use inline syscalls, but
if so the function-call-based implementations should be added back in
a unified way for all archs.
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the reordering of headers caused some risc archs to not see
the __syscall declaration anymore.
this caused build errors on mips with any compiler,
and on arm and microblaze with clang.
we now declare it locally just like the powerpc port does.
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the issue at hand is that many syscalls require as an argument the
kernel-ABI size of sigset_t, intended to allow the kernel to switch to
a larger sigset_t in the future. previously, each arch was defining
this size in syscall_arch.h, which was redundant with the definition
of _NSIG in bits/signal.h. as it's used in some not-quite-portable
application code as well, _NSIG is much more likely to be recognized
and understood immediately by someone reading the code, and it's also
shorter and less cluttered.
note that _NSIG is actually 65/129, not 64/128, but the division takes
care of throwing away the off-by-one part.
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no problems were detected so far, but the constraints seem to have
been invalid just like the mips ones.
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most pure-syscall-wrapper functions compile to the smallest/simplest
code possible (save r7 ; load syscall # ; svc 0 ; restore r7 ; tail
call to __syscall_ret).
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now public syscall.h only exposes __NR_* and SYS_* constants and the
variadic syscall function. no macros or inline functions, no
__syscall_ret or other internal details, no 16-/32-bit legacy syscall
renaming, etc. this logic has all been moved to src/internal/syscall.h
with the arch-specific parts in arch/$(ARCH)/syscall_arch.h, and the
amount of arch-specific stuff has been reduced to a minimum.
changes still need to be reviewed/double-checked. minimal testing on
i386 and mips has already been performed.
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