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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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in the case where a non-symlink file was replaced by a symlink during
the fchmodat operation with AT_SYMLINK_NOFOLLOW, mode change on the
new symlink target was successfully suppressed, but the error was not
reported. instead, fchmodat simply returned 0.
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such archs are expected to omit definitions of the SYS_* macros for
syscalls their kernels lack from arch/$ARCH/bits/syscall.h. the
preprocessor is then able to select the an appropriate implementation
for affected functions. two basic strategies are used on a
case-by-case basis:
where the old syscalls correspond to deprecated library-level
functions, the deprecated functions have been converted to wrappers
for the modern function, and the modern function has fallback code
(omitted at the preprocessor level on new archs) to make use of the
old syscalls if the new syscall fails with ENOSYS. this also improves
functionality on older kernels and eliminates the incentive to program
with deprecated library-level functions for the sake of compatibility
with older kernels.
in other situations where the old syscalls correspond to library-level
functions which are not deprecated but merely lack some new features,
such as the *at functions, the old syscalls are still used on archs
which support them. this may change at some point in the future if or
when fallback code is added to the new functions to make them usable
(possibly with reduced functionality) on old kernels.
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I intend to add more Linux workarounds that depend on using these
pathnames, and some of them will be in "syscall" functions that, from
an anti-bloat standpoint, should not depend on the whole snprintf
framework.
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previously, the AT_SYMLINK_NOFOLLOW flag was ignored, giving
dangerously incorrect behavior -- the target of the symlink had its
modes changed to the modes (usually 0777) intended for the symlink).
this issue was amplified by the fact that musl provides lchmod, as a
wrapper for fchmodat, which some archival programs take as a sign that
symlink modes are supported and thus attempt to use.
emulating AT_SYMLINK_NOFOLLOW was a difficult problem, and I
originally believed it could not be solved, at least not without
depending on kernels newer than 3.5.x or so where O_PATH works halfway
well. however, it turns out that accessing O_PATH file descriptors via
their pseudo-symlink entries in /proc/self/fd works much better than
trying to use the fd directly, and works even on older kernels.
moreover, the kernel has permanently pegged these references to the
inode obtained by the O_PATH open, so there should not be race
conditions with the file being moved, deleted, replaced, etc.
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