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Normally, in static builds, the first code that runs is _start, in e.g. sysdeps/x86_64/start.S, which quickly calls __libc_start_main, passing it the argv etc. Among the first things __libc_start_main does is initializing the tunables (based on env), then CPU features, and then calls _dl_relocate_static_pie (). Specifically, this runs ifunc resolvers to pick, based on the CPU features discovered earlier, the most suitable implementation of "string" functions such as memcpy. Before that point, calling memcpy (or other ifunc-resolved functions) will not work. In the Hurd port, things are more complex. In order to get argv/env for our process, glibc normally needs to do an RPC to the exec server, unless our args/env are already located on the stack (which is what happens to bootstrap processes spawned by GNU Mach). Fetching our argv/env from the exec server has to be done before the call to __libc_start_main, since we need to know what our argv/env are to pass them to __libc_start_main. On the other hand, the implementation of the RPC (and other initial setup needed on the Hurd before __libc_start_main can be run) is not very trivial. In particular, it may (and on x86_64, will) use memcpy. But as described above, calling memcpy before __libc_start_main can not work, since the GOT entry for it is not yet initialized at that point. Work around this by pre-filling the GOT entry with the baseline version of memcpy, __memcpy_sse2_unaligned. This makes it possible for early calls to memcpy to just work. The initial value of the GOT entry is unused on x86_64, and changing it won't interfere with the relocation being performed later: once _dl_relocate_static_pie () is called, the baseline version will get replaced with the most suitable one, and that is what subsequent calls of memcpy are going to call. Checked on x86_64-gnu. Signed-off-by: Sergey Bugaev <bugaevc@gmail.com> Message-Id: <20230429201822.2605207-6-bugaevc@gmail.com>
This directory contains the sources of the GNU C Library. See the file "version.h" for what release version you have. The GNU C Library is the standard system C library for all GNU systems, and is an important part of what makes up a GNU system. It provides the system API for all programs written in C and C-compatible languages such as C++ and Objective C; the runtime facilities of other programming languages use the C library to access the underlying operating system. In GNU/Linux systems, the C library works with the Linux kernel to implement the operating system behavior seen by user applications. In GNU/Hurd systems, it works with a microkernel and Hurd servers. The GNU C Library implements much of the POSIX.1 functionality in the GNU/Hurd system, using configurations i[4567]86-*-gnu. When working with Linux kernels, this version of the GNU C Library requires Linux kernel version 3.2 or later. Also note that the shared version of the libgcc_s library must be installed for the pthread library to work correctly. The GNU C Library supports these configurations for using Linux kernels: aarch64*-*-linux-gnu alpha*-*-linux-gnu arc*-*-linux-gnu arm-*-linux-gnueabi csky-*-linux-gnuabiv2 hppa-*-linux-gnu i[4567]86-*-linux-gnu x86_64-*-linux-gnu Can build either x86_64 or x32 ia64-*-linux-gnu loongarch64-*-linux-gnu Hardware floating point, LE only. m68k-*-linux-gnu microblaze*-*-linux-gnu mips-*-linux-gnu mips64-*-linux-gnu or1k-*-linux-gnu powerpc-*-linux-gnu Hardware or software floating point, BE only. powerpc64*-*-linux-gnu Big-endian and little-endian. s390-*-linux-gnu s390x-*-linux-gnu riscv32-*-linux-gnu riscv64-*-linux-gnu sh[34]-*-linux-gnu sparc*-*-linux-gnu sparc64*-*-linux-gnu If you are interested in doing a port, please contact the glibc maintainers; see https://www.gnu.org/software/libc/ for more information. See the file INSTALL to find out how to configure, build, and install the GNU C Library. You might also consider reading the WWW pages for the C library at https://www.gnu.org/software/libc/. The GNU C Library is (almost) completely documented by the Texinfo manual found in the `manual/' subdirectory. The manual is still being updated and contains some known errors and omissions; we regret that we do not have the resources to work on the manual as much as we would like. For corrections to the manual, please file a bug in the `manual' component, following the bug-reporting instructions below. Please be sure to check the manual in the current development sources to see if your problem has already been corrected. Please see https://www.gnu.org/software/libc/bugs.html for bug reporting information. We are now using the Bugzilla system to track all bug reports. This web page gives detailed information on how to report bugs properly. The GNU C Library is free software. See the file COPYING.LIB for copying conditions, and LICENSES for notices about a few contributions that require these additional notices to be distributed. License copyright years may be listed using range notation, e.g., 1996-2015, indicating that every year in the range, inclusive, is a copyrightable year that would otherwise be listed individually.