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gthread: Fix futex timespec type on 32-bit kernels with 64-bit userspace

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The `struct timespec` type documented as being passed to the `futex()`
syscall actually needs to be the *kernel’s* timespec type. This will be
a different width from the userspace timespec type if running a 64-bit
userspace on a 32-bit kernel.

That mismatch will cause `g_cond_wait_until()` to return `FALSE`
immediately.

No other uses of `futex()` in GLib use the timeout argument, so they’re
all OK.

Following a detailed suggestion by Rich Felker, pass a different
timespec type into `futex()` if `__NR_futex_time64` is defined. That’s
the 64-bit time version of `futex()` which was added in kernel 5.1, and
which was only added for 32-bit kernels.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #2634
This commit is contained in:
Philip Withnall 2022-04-26 11:50:23 +01:00
parent 606bdcdb18
commit eec65c761b
1 changed files with 32 additions and 1 deletions
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33
glib/gthread-posix.c
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@ -1599,6 +1599,13 @@ g_cond_wait_until (GCond *cond,
{
struct timespec now;
struct timespec span;
#ifdef __NR_futex_time64
long span_arg[2];
G_STATIC_ASSERT (sizeof (span_arg[0]) == 4);
#else
struct timespec span_arg;
#endif
guint sampled;
int res;
gboolean success;
@ -1618,9 +1625,33 @@ g_cond_wait_until (GCond *cond,
if (span.tv_sec < 0)
return FALSE;
/* On x32 (ILP32 ABI on x86_64) and potentially sparc64, the raw futex()
* syscall takes a 32-bit timespan argument *regardless* of whether userspace
* is using 32-bit or 64-bit `struct timespec`. This means that we can’t
* unconditionally pass a `struct timespec` pointer into the syscall.
*
* Assume that any such platform is new enough to define the
* `__NR_futex_time64` workaround syscall (which accepts 64-bit timespecs,
* introduced in kernel 5.1), and use that as a proxy for whether to pass in
* `long[2]` or `struct timespec`.
*
* As per https://lwn.net/Articles/776427/, the `time64` syscalls only exist
* on 32-bit platforms, so in this case `sizeof(long)` should always be
* 32 bits.
*
* Don’t bother actually calling `__NR_futex_time64` as the `span` is relative
* and hence very unlikely to overflow, even if using 32-bit longs.
*/
#ifdef __NR_futex_time64
span_arg[0] = span.tv_sec;
span_arg[1] = span.tv_nsec;
#else
span_arg = span;
#endif
sampled = cond->i[0];
g_mutex_unlock (mutex);
res = syscall (__NR_futex, &cond->i[0], (gsize) FUTEX_WAIT_PRIVATE, (gsize) sampled, &span);
res = syscall (__NR_futex, &cond->i[0], (gsize) FUTEX_WAIT_PRIVATE, (gsize) sampled, &span_arg);
success = (res < 0 && errno == ETIMEDOUT) ? FALSE : TRUE;
g_mutex_lock (mutex);