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syscall.h was chosen as the header to declare it, since its intended
usage is alongside syscalls as a fallback for operations the direct
syscall does not support.
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this code path is used only on archs without the plain, non-at
syscalls, and only when the fstat syscall fails with EBADF on a valid
file descriptor. this in turn can happen only for O_PATH file
descriptors, and may not happen at all on the newer kernels needed for
supporting such archs.
with the flags argument omitted, spurious fstat failures may happen
when the argument register happens to have the AT_SYMLINK_NOFOLLOW bit
set.
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the workaround/fallback code for supporting O_PATH file descriptors
when the kernel lacks support for performing these operations on them
caused EBADF to get replaced by ENOENT (due to missing entry in
/proc/self/fd). this is unlikely to affect real-world code (calls that
might yield EBADF are generally unsafe, especially in library code)
but it was breaking some test cases.
the fix I've applied is something of a tradeoff: it adds one syscall
to these operations on kernels where the workaround is needed. the
alternative would be to catch ENOENT from the /proc lookup and
translate it to EBADF, but I want to avoid doing that in the interest
of not touching/depending on /proc at all in these functions as long
as the kernel correctly supports the operations. this is following the
general principle of isolating hacks to code paths that are taken on
broken systems, and keeping the code for correct systems completely
hack-free.
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on newer kernels, fchdir and fstat work anyway. this same fix should
be applied to any other syscalls that are similarly affected.
with this change, the current definitions of O_SEARCH and O_EXEC as
O_PATH are mostly conforming to POSIX requirements. the main remaining
issue is that O_NOFOLLOW has different semantics.
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