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Crossing 2GB boundaries with indirect calls and jumps can use more branch prediction resources on Intel Golden Cove CPU (see the "Misprediction for Branches >2GB" section in Intel 64 and IA-32 Architectures Optimization Reference Manual.) There is visible performance improvement on workloads with many PLT calls when executable and shared libraries are mmapped below 2GB. Add the Prefer_MAP_32BIT_EXEC bit so that mmap will try to map executable or denywrite pages in shared libraries with MAP_32BIT first. NB: Prefer_MAP_32BIT_EXEC reduces bits available for address space layout randomization (ASLR), which is always disabled for SUID programs and can only be enabled by the tunable, glibc.cpu.prefer_map_32bit_exec, or the environment variable, LD_PREFER_MAP_32BIT_EXEC. This works only between shared libraries or between shared libraries and executables with addresses below 2GB. PIEs are usually loaded at a random address above 4GB by the kernel.
43 lines
1.5 KiB
C
43 lines
1.5 KiB
C
/* Linux mmap system call. x86-64 version.
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Copyright (C) 2015-2023 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 License as
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published by the Free Software Foundation; either version 2.1 of the
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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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#ifndef MMAP_X86_64_INTERNAL_H
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#define MMAP_X86_64_INTERNAL_H
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#include <ldsodefs.h>
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/* If the Prefer_MAP_32BIT_EXEC bit is set, try to map executable or
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denywrite pages with MAP_32BIT first. */
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#define MMAP_PREPARE(addr, len, prot, flags, fd, offset) \
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if ((addr) == NULL \
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&& (((prot) & PROT_EXEC) != 0 \
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|| ((flags) & MAP_DENYWRITE) != 0) \
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&& HAS_ARCH_FEATURE (Prefer_MAP_32BIT_EXEC)) \
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{ \
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void *ret = (void*) INLINE_SYSCALL_CALL (mmap, (addr), (len), \
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(prot), \
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(flags) | MAP_32BIT, \
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(fd), (offset)); \
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if (ret != MAP_FAILED) \
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return ret; \
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}
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#include_next <mmap_internal.h>
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#endif
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