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originally the namespace-infringing "large file support" interfaces
were included as part of glibc-ABI-compat, with the intent that they
not be used for linking, since our off_t is and always has been
unconditionally 64-bit and since we usually do not aim to support
nonstandard interfaces when there is an equivalent standard interface.
unfortunately, having the symbols present and available for linking
caused configure scripts to detect them and attempt to use them
without declarations, producing all the expected ill effects that
entails.
as a result, commit 2dd8d5e1b8ba1118ff1782e96545cb8a2318592c was made
to prevent this, using macros to redirect the LFS64 names to the
standard names, conditional on _GNU_SOURCE or _LARGEFILE64_SOURCE.
however, this has turned out to be a source of further problems,
especially since g++ defines _GNU_SOURCE by default. in particular,
the presence of these names as macros breaks a lot of valid code.
this commit removes all the LFS64 symbols and replaces them with a
mechanism in the dynamic linker symbol lookup failure path to retry
with the spurious "64" removed from the symbol name. in the future,
if/when the rest of glibc-ABI-compat is moved out of libc, this can be
removed.
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this avoids the need for implementation-internal callers to depend on
the nonstandard AT_EMPTY_PATH extension to use __fstatat and isolates
knowledge of that extension to the implementation of __fstat.
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these functions cannot provide the glibc lfs64-ABI-compatible symbols
when time_t differs from what it was in that ABI. instead, the aliases
need to be provided by the time32 compat shims or through some other
mechanism.
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AT_FDCWD is not a valid file descriptor, so POSIX requires fstat to
fail with EBADF. if passed to fstatat, the call would spuriously
succeed and return results for the working directory.
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equivalent logic for fstat+O_PATH fallback and direct use of
stat/lstat syscalls where appropriate is kept, now in the fstatat
function. this change both improves functionality (now, fstatat forms
equivalent to fstat/lstat/stat will work even on kernels too old to
have the at functions) and localizes direct interfacing with the
kernel stat structure to one file.
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the LFS64 macro was not self-documenting and barely saved any
characters. simply use weak_alias directly so that it's clear what's
being done, and doesn't depend on a header to provide a strange macro.
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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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