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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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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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reading the variadic mode argument is only valid when the O_CREAT flag
is present. this probably does not matter, but is needed for formal
correctness, and could affect LTO or other full-program analysis.
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since there is no easy way to detect whether open honored or ignored
the O_CLOEXEC flag, the optimal solution to providing a fallback is
simply to make the fcntl syscall to set the close-on-exec flag
immediately after open returns.
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open is handled specially because it is used from so many places, in
so many variants (2 or 3 arguments, setting errno or not, and
cancellable or not). trying to do it as a function would not only
increase bloat, but would also risk subtle breakage.
this is the first step towards supporting "new" archs where linux
lacks "old" syscalls.
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this patch improves the correctness, simplicity, and size of
cancellation-related code. modulo any small errors, it should now be
completely conformant, safe, and resource-leak free.
the notion of entering and exiting cancellation-point context has been
completely eliminated and replaced with alternative syscall assembly
code for cancellable syscalls. the assembly is responsible for setting
up execution context information (stack pointer and address of the
syscall instruction) which the cancellation signal handler can use to
determine whether the interrupted code was in a cancellable state.
these changes eliminate race conditions in the previous generation of
cancellation handling code (whereby a cancellation request received
just prior to the syscall would not be processed, leaving the syscall
to block, potentially indefinitely), and remedy an issue where
non-cancellable syscalls made from signal handlers became cancellable
if the signal handler interrupted a cancellation point.
x86_64 asm is untested and may need a second try to get it right.
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