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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>
31 lines
1.0 KiB
ArmAsm
31 lines
1.0 KiB
ArmAsm
/* Startup code for statically linked Hurd/x86_64 binaries.
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Copyright (C) 1998-2023 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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.text
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.globl _start
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_start:
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leaq __memcpy_sse2_unaligned(%rip), %rax
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movq %rax, memcpy@GOTPCREL(%rip)
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call _hurd_stack_setup
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xorq %rdx, %rdx
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jmp _start1
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#define _start _start1
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#include <sysdeps/x86_64/start.S>
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