for conformance, two functions should not have the same address. a
conforming program could use the addresses of getc and fgetc in ways
that assume they are distinct. normally i would just use a wrapper,
but these functions are so small and performance-critical that an
extra layer of function call could make the one that's a wrapper
nearly twice as slow, so I'm just duplicating the code instead.
-lpcc only works if -nostdlib is not passed, so it's useless. instead,
use -print-file-name to look up the full pathname for libpcc.a, and
check whether that succeeds before trying to link with the result.
also, silence pcc's junk printed on stdout during tests.
in old versions of pcc, the directory containing libpcc.a was not in
the library path, and other options like -print-file-name may have
been needed to locate it. however, -print-file-name itself seems to
have been added around the same time that the directory was added to
the search path, and moreover, I see no evidence that older versions
of pcc are capable of building a working musl shared library. thus, it
seems reasonable to just test whether -lpcc is accepted.
on x86 and some other archs, functions which make function calls which
might go through a PLT incur a significant overhead cost loading the
GOT register prior to making the call. this load is utterly useless in
musl, since all calls are bound at library-creation time using
-Bsymbolic-functions, but the compiler has no way of knowing this, and
attempts to set the default visibility to protected have failed due to
bugs in GCC and binutils.
this commit simply manually assigns hidden/protected visibility, as
appropriate, to a few internal-use-only functions which have many
callers, or which have callers that are hot paths like getc/putc. it
shaves about 5k off the i386 libc.so with -Os. many of the
improvements are in syscall wrappers, where the benefit is just size
and performance improvement is unmeasurable noise amid the syscall
overhead. however, stdio may be measurably faster.
if in the future there are toolchains that can do the same thing
globally without introducing linking bugs, it might be worth
considering removing these workarounds.
pcc wrongly passes any option beginning with -m to the linker, and
will break at link time if these options were added to CFLAGS. testing
linking lets us catch this at configure time and skip them.
these functions must behave as if they obtain the lock via flockfile
to satisfy POSIX requirements. since another thread can provably hold
the lock when they are called, they must wait to obtain the lock
before they can return, even if the correct return value could be
obtained without locking. in the case of fclose and freopen, failure
to do so could cause correct (albeit obscure) programs to crash or
otherwise misbehave; in the case of feof, ferror, and fwide, failure
to obtain the lock could sometimes return incorrect results. in any
case, having these functions proceed and return while another thread
held the lock was wrong.
1. don't open /dev/null just as a basis to copy flags; use shared
__fmodeflags function to get the right file flags for the mode.
2. handle the case (probably invalid, but whatever) case where the
original stream's file descriptor was closed; previously, the logic
re-closed it.
3. accept the "e" mode flag for close-on-exec; update dup3 to fallback
to using dup2 so we can simply call __dup3 instead of putting fallback
logic in freopen itself.
the behavior of putenv is left undefined if the argument does not
contain an equal sign, but traditional implementations behave this way
and gnulib replaces putenv if it doesn't do this.
__release_ptc() is only valid in the parent; if it's performed in the
child, the lock will be unlocked early then double-unlocked later,
corrupting the lock state.
since we target systems without overcommit, special care should be
taken that system() and popen(), like posix_spawn(), do not fail in
processes whose commit charges are too high to allow ordinary forking.
this in turn requires special precautions to ensure that the parent
process's signal handlers do not end up running in the shared-memory
child, where they could corrupt the state of the parent process.
popen has also been updated to use pipe2, so it does not have a
fd-leak race in multi-threaded programs. since pipe2 is missing on
older kernels, (non-atomic) emulation has been added.
some silly bugs in the old code should be gone too.
despite documentation that makes it sound a lot different, the only
ABI-constraint difference between TLS variants II and I seems to be
that variant II stores the initial TLS segment immediately below the
thread pointer (i.e. the thread pointer points to the end of it) and
variant I stores the initial TLS segment above the thread pointer,
requiring the thread descriptor to be stored below. the actual value
stored in the thread pointer register also tends to have per-arch
random offsets applied to it for silly micro-optimization purposes.
with these changes applied, TLS should be basically working on all
supported archs except microblaze. I'm still working on getting the
necessary information and a working toolchain that can build TLS
binaries for microblaze, but in theory, static-linked programs with
TLS and dynamic-linked programs where only the main executable uses
TLS should already work on microblaze.
alignment constraints have not yet been heavily tested, so it's
possible that this code does not always align TLS segments correctly
on archs that need TLS variant I.
usage of vfork creates a situation where a process of lower privilege
may momentarily have write access to the memory of a process of higher
privilege.
consider the case of a multi-threaded suid program which is calling
posix_spawn in one thread while another thread drops the elevated
privileges then runs untrusted (relative to the elevated privilege)
code as the original invoking user. this untrusted code can then
potentially modify the data the child process will use before calling
exec, for example changing the pathname or arguments that will be
passed to exec.
note that if vfork is implemented as fork, the lock will not be held
until the child execs, but since memory is not shared it does not
matter.
with this change, pcc-built musl libc.so seems to work correctly. the
problem is that pcc generates GOT lookups for external-linkage symbols
even if they are hidden, rather than using GOT-relative addressing.
the entire reason we're using hidden visibility on the __libc object
is to make it accessible prior to relocations -- not to mention
inexpensive to access. unfortunately, the workaround makes it even
more expensive on pcc.
when the pcc issue is fixed, an appropriate version test should be
added so new pcc can use the much more efficient variant.
this is actually a rather subtle issue: do arrays decay to pointers
when used as inline asm args? gcc says yes, but currently pcc says no.
hopefully this discrepency in pcc will be fixed, but since the
behavior is not clearly defined anywhere I can find, I'm using an
explicit operation to cause the decay to occur.
this makes it so the #undef libc and __libc name are no longer needed,
which were problematic because the "accessor function" mode for
accessing the libc struct could not be used, breaking build on any
compiler without (working) visibility.
this is necessary because posix_spawn calls sigaction after vfork, and
if the thread pointer is not already initialized, initializing it in
the child corrupts the parent process's state.
this doubles the performance of the fastest syscalls on the atom I
tested it on; improvement is reportedly much more dramatic on
worst-case cpus. cannot be used for cancellable syscalls.
the code in __libc_start_main is now responsible for parsing auxv,
rather than duplicating the parsing all over the place. this should
shave off a few cycles and some code size. __init_libc is left as an
external-linkage function despite the fact that it could be static, to
prevent it from being inlined and permanently wasting stack space when
main is called.
a few other minor changes are included, like eliminating per-thread
ssp canaries (they were likely broken when combined with certain
dlopen usages, and completely unnecessary) and some other unnecessary
checks. since this code gets linked into every program, it should be
as small and simple as possible.
at initial program load, all libraries must be loaded before the
thread pointer can be setup, since the TP-relative addresses of all
initial TLS objects must be constant.
this is needed to ensure async-cancel-safety, i.e. to make it safe to
access TLS objects when async cancellation is enabled. otherwise, if
cancellation were acter upon after the atomic fetch/add but before the
thread saved the obtained memory, another access to the same TLS in
the cancellation handler could end up performing the atomic fetch/add
again, consuming more memory than is actually available and
overflowing into other objects on the heap.
Rich Felker
nsz