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it's been reported that the vdso clock_gettime64 function on (32-bit)
arm is broken, producing erratic results that grow at a rate far
greater than one reported second per actual elapsed second. the vdso
function seems to have been added sometime between linux 5.4 and 5.6,
so if there's ever been a working version, it was only present for a
very short window.
it's not clear what the eventual upstream kernel solution will be, but
something needs to be done on the libc side so as not to be producing
binaries that seem to work on older/existing/lts kernels (which lack
the function and thus lack the bug) but will break fantastically when
moving to newer kernels.
hopefully vdso support will be added back soon, but with a new symbol
name or version from the kernel to allow continued rejection of broken
ones.
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analogous to commit b287cd745c2243f8e5114331763a5a9813b5f6ee but for
the custom FILE stream type the wcstol and wcstod family use. __toread
could be used here as well, but there's a simple direct fix to make
the buffer pointers initially valid for subtraction, so just do that
to avoid pulling in stdio exit code in programs that don't use stdio.
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the sh version of fesetround or'd the new rounding mode onto the
control register without clearing the old rounding mode bits, making
changes sticky. this was the root cause of multiple test failures.
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apparently this function was intended at some point to be used by
strto* family as well, and thus was put in its own file; however, as
far as I can tell, it's only ever been used by vsscanf. move it to the
same file to reduce the number of source files and external symbols.
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this idea came up when I thought we might need to zero the UNGET
portion of buf as well, but it seems like a useful improvement even
when that turned out not to be necessary.
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shgetc sets up to be able to perform an "unget" operation without the
caller having to remember and pass back the character value, and for
this purpose used a conditional store idiom:
if (f->rpos[-1] != c) f->rpos[-1] = c
to make it safe to use with non-writable buffers (setup by the
sh_fromstring macro or __string_read with sscanf).
however, validity of this depends on the buffer space at rpos[-1]
being initialized, which is not the case under some conditions
(including at least unbuffered files and fmemopen ones).
whenever data was read "through the buffer", the desired character
value is already in place and does not need to be written. thus,
rather than testing for the absence of the value, we can test for
rpos<=buf, indicating that the last character read could not have come
from the buffer, and thereby that we have a "real" buffer (possibly of
zero length) with writable pushback (UNGET bytes) below it.
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as reported/analyzed by Pascal Cuoq, the shlim and shcnt
macros/functions are called by the scanf core (vfscanf) with f->rpos
potentially null (if the FILE is not yet activated for reading at the
time of the call). in this case, they compute differences between a
null pointer (f->rpos) and a non-null one (f->buf), resulting in
undefined behavior.
it's unlikely that any observably wrong behavior occurred in practice,
at least without LTO, due to limits on what's visible to the compiler
from translation unit boundaries, but this has not been checked.
fix is simply ensuring that the FILE is activated for read mode before
entering the main scanf loop, and erroring out early if it can't be.
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