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this change is necessary or pthread_create will always fail on
security-hardened kernels. i considered first trying to make the stack
executable and simply retrying without execute permissions when the
first try fails, but (1) this would incur a serious performance
penalty on hardened systems, and (2) having the stack be executable is
just a bad idea from a security standpoint.
if there is real-world "GNU C" code that uses nested functions with
threads, and it can't be fixed, we'll have to consider other ways of
solving the problem, but for now this seems like the best fix.
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these new rules should avoid spurious error messages when the
directory (usually /lib) and the dynamic linker symlink already exist,
and minimize the spam when they can't be created.
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old: 2*atan2(sqrt(1-x),sqrt(1+x))
new: atan2(fabs(sqrt((1-x)*(1+x))),x)
improvements:
* all edge cases are fixed (sign of zero in downward rounding)
* a bit faster (here a single call is about 131ns vs 162ns)
* a bit more precise (at most 1ulp error on 1M uniform random
samples in [0,1), the old formula gave some 2ulp errors as well)
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this could lead to spurious failures of wide printf functions
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musl does not support legacy 32-bit-off_t whatsoever. off_t is always
64 bit, and correct programs that use off_t and the standard functions
will just work out of the box. (on glibc, they would require
-D_FILE_OFFSET_BITS=64 to work.) however, some programs instead define
_LARGEFILE64_SOURCE and use alternate versions of all the standard
types and functions with "64" appended to their names.
we do not want code to actually get linked against these functions
(it's ugly and inconsistent), so macros are used instead of prototypes
with weak aliases in the library itself. eventually the weak aliases
may be added at the library level for the sake of using code that was
originally built against glibc, but the macros will still be the
desired solution in the headers.
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these actually work, but for now they prohibit actually setting
priority levels and report min/max priority as 0.
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in6_* is in the reserved namespace, so this is valid
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pthread structure has been adjusted to match the glibc/GCC abi for
where the canary is stored on i386 and x86_64. it will need variants
for other archs to provide the added security of the canary's entropy,
but even without that it still works as well as the old "minimal" ssp
support. eventually such changes will be made anyway, since they are
also needed for GCC/C11 thread-local storage support (not yet
implemented).
care is taken not to attempt initializing the thread pointer unless
the program actually uses SSP (by reference to __stack_chk_fail).
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hopefully the annoyance of this will be minimal. these files all
define internal interfaces which can change at any time; if different
modules are using different versions of the interfaces, the library
will badly break. ideally we would scan and add the dependency only
for C files that actually reference the affected interfaces, but for
now, err on the side of caution and force a rebuild of everything if
any of them have changed.
this commit is in preparation for the upcoming ssp overhaul commit,
which will change internals of the pthread struct.
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looks like nik copied these "extra arguments" from the i386 code.
they're not actually arguments there, just 1-byte instructions to
make sure the stack is aligned to 16 bytes after all the other
arguments are pushed. since each push is 8 bytes on x86_64, they
happened to have no effect here, but their presence is confusing and a
minor waste of space.
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it does not work; after further consideration, a separate Scrt1.s for
pie really is essential. it would be nice if the unified approach
worked, but the linker fails to generate the correct PLT entries and
instead puts textrels in the main program, which don't work because
the kernel maps the text read-only.
new Scrt1.s will be committed soon in place of this.
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these are POSIX 2008 (previously GNU extension) functions that are
rarely used. apparently they had never been tested before, since the
end-of-string logic was completely missing. mbsnrtowcs is used by
modern versions of bash for its glob implementation, and and this bug
was causing tab completion to hang in an infinite loop.
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these were at best of limited usefulness (for bootstrapping new
systems, mainly) and at worst caused real kernel headers to get
overwritten when upgrading libc.
in case they're needed by anyone, the exact same files are now
available in a new git repository:
git://git.etalabs.net/mini-lkh
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the major change here is that CFLAGS is now a variable that can be
changed entirely under user control, without causing essential flags
to be lost. previously, "CFLAGS += ..." was valid in config.mak, but
using "CFLAGS = ..." in config.mak would have badly broken the build
process unless the user took care to copy the necessary flags out of
the main Makefile.
I have also added a distclean target that removes config.mak.
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as far as I can tell, it's not useful and never way. I wrote it way
back under the assumption that non-weak symbols in the POSIX or
extension namespace could conflict with legitimate uses of the same
symbol name in the main program or other libraries, but that does not
seem to be the case.
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this is a nonstandard function so it's not clear what conditions it
should satisfy. my intent is that it be fast and exact for positive
integral exponents when the result fits in the destination type, and
fast and correctly rounded for small negative integral exponents.
otherwise we aim for at most 1ulp error; it seems to differ from pow
by at most 1ulp and it's often 2-5 times faster than pow.
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this caused misreading of certain floating point values that are exact
multiples of large powers of ten, unpredictable depending on prior
stack contents.
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untested
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unlike the old one, this one's algorithm does not suffer from
potential stack overflow issues or pathologically bad performance on
certain patterns. instead of backtracking, it uses a matching
algorithm which I have not seen before (unsure whether I invented or
re-invented it) that runs in O(1) space and O(nm) time. it may be
possible to improve the time to O(n), but not without significantly
greater complexity.
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if the compiler provides a value, use it; otherwise fallback to the
platform default (2).
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an invalid bracket expression must be treated as if the opening
bracket were just a literal character. this is to fix a bug whereby
POSIX left the behavior of the "[" shell command undefined due to it
being an invalid bracket expression.
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provide the minimal level of dynamic linker-to-debugger glue needed to
let gdb find loaded libraries and load their symbols.
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the code is written to pre-init the thread pointer in static linked
programs that pull in __stack_chk_fail or dynamic-linked programs that
lookup the symbol. no explicit canary is set; the canary will be
whatever happens to be in the thread structure at the offset gcc
hard-coded. this can be improved later.
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otherwise this BADLY breaks if -funsigned-char is passed to gcc
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i did some testing trying to switch malloc to use the new internal
lock with priority inheritance, and my malloc contention test got
20-100 times slower. if priority inheritance futexes are this slow,
it's simply too high a price to pay for avoiding priority inversion.
maybe we can consider them somewhere down the road once the kernel
folks get their act together on this (and perferably don't link it to
glibc's inefficient lock API)...
as such, i've switch __lock to use malloc's implementation of
lightweight locks, and updated all the users of the code to use an
array with a waiter count for their locks. this should give optimal
performance in the vast majority of cases, and it's simple.
malloc is still using its own internal copy of the lock code because
it seems to yield measurably better performance with -O3 when it's
inlined (20% or more difference in the contention stress test).
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this bug probably would have gone unnoticed since it's only used in
the fallback code for systems where priority-inheritance locking
fails. unfortunately this approach results in one spurious wake
syscall on the final unlock, when there are no waiters remaining. the
alternative (possibly better) would be to use broadcast wakes instead
of reflagging the waiter unconditionally, and let each waiter reflag
itself; this saves one syscall at the expense of invoking the
"thundering herd" effect (worse performance degredation) when there
are many waiters.
ideally we would be able to update all of our locks to use an array of
two ints rather than a single int, and use a separate counter system
like proper mutexes use; then we could avoid all spurious wake calls
without resorting to broadcasts. however, it's not clear to me that
priority inheritance futexes support this usage. the kernel sets the
waiters flag for them (just like we're doing now) and i can't tell if
it's safe to bypass the kernel when unlocking just because we know
(from private data, the waiter count) that there are no waiters. this
is something that could be explored in the future.
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we use priority inheritance futexes if possible so that the library
cannot hit internal priority inversion deadlocks in the presence of
realtime priority scheduling (full support to be added later).
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i tried to go with improving the old binary-search-based algorithm,
but between growth in the number of ranges, bad performance, and lack
of confidence in the binary search code's stability under changes in
the table, i decided it was worth the extra 1.8k to have something
clean and maintainable.
also note that, like the alpha and punct tables, there's definitely
room to optimize the nonspacing/wide tables by overlapping subtables.
this is not a high priority, but i've begun looking into how to do it,
and i suspect the table sizes can be roughly halved. if that turns out
to be true, the new, fast, table-based implementation will be roughly
the same size as if i had just extended the old binary search one.
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also special-case ß (U+00DF) as lowercase even though it does not have
a mapping to uppercase. unicode added an uppercase version of this
character but does not map it, presumably because the uppercase
version is not actually used except for some obscure purpose...
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this happened due to their entries in UnicodeData.txt
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alpha is defined as unicode property "Alphabetic" plus category Nd
minus ASCII digits minus 2 special-cased Thai punctuation marks
supposedly misclassified by Unicode as letters.
punct is defined as all of unicode except control, alphanumeric, and
space characters.
the tables were generated by a simple tool based on the code posted
previously to the mailing list. in the future, this and other code
used for maintaining locale/iconv/i18n data will be published either
in the main source repository or in a separate locale data generation
repository.
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note that dlerror is specified to be non-thread-safe, so no locking is
performed on the error flag or message aside from the rwlock already
held by dlopen or dlsym. if 2 invocations of dlsym are generating
errors at the same time, they could clobber each other's results, but
the resulting string, albeit corrupt, will still be null-terminated.
any use of dlerror in such a situation could not be expected to give
meaningful results anyway.
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