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// SPDX-License-Identifier: GPL-2.0
/*
* An x86 alternative whose replacement code is part of the patched function's
* checksum.
*
* checksum_update_insn() walks insn->alts after hashing the instruction
* itself, hashing the alternative's type and, when the replacement forms a
* group, its feature number and every instruction in it. So editing only the
* replacement -- code the CPU may or may not ever run -- has to move the
* function's checksum.
*
* It is reached through objtool's own alternative handling, so the object has
* to go through the check pass first: insn->alts is built there, not by the
* compiler.
*
* struct alt_instr is written out by hand as in empty_alternative.c: s32
* instr_offset, s32 repl_offset, u32 ft_flags, u8 instrlen, u8 replacementlen.
*
* Variants, applied to the patched build only:
*
* ALT_REPL the replacement instruction changes; the original does not
* ALT_FEATURE the feature number changes; no code changes at all
*/
static const char __modinfo[]
__attribute__((section(".modinfo"), used, aligned(1))) = "\0name=vmlinux";
/*
* Both spellings are two bytes, because a replacement may not be longer than
* the instruction it replaces: "xchg %ax, %ax" is 66 90 and two nops are
* 90 90. The original below is padded to match.
*/
#if defined(PATCHED) && defined(ALT_REPL)
#define REPL_INSN " nop\n\t nop\n\t"
#else
#define REPL_INSN " xchg %ax, %ax\n\t"
#endif
#if defined(PATCHED) && defined(ALT_FEATURE)
#define FEATURE "7"
#else
#define FEATURE "3"
#endif
int target(int x)
{
asm volatile(
"661: nop\n\t"
" nop\n\t"
"662:\n\t"
".pushsection .altinstr_replacement, \"ax\"\n\t"
".globl target_repl\n\t"
"target_repl:\n\t"
REPL_INSN
"target_repl_end:\n\t"
".popsection\n\t"
".pushsection .altinstructions, \"aM\", @progbits, 14\n\t"
".long 661b - .\n\t"
".long target_repl - .\n\t"
".long " FEATURE "\n\t"
".byte 662b - 661b\n\t"
".byte target_repl_end - target_repl\n\t"
".popsection\n\t");
return x + 1;
}
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