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9dba9c67e5
Commit in Fixes moved the optimize_nops() call inside apply_relocation()
and made it a second optimization pass after the relocations have been
done.
Since optimize_nops() works only on NOPs, that is fine and it'll simply
jump over instructions which are not NOPs.
However, it made that call with repl_len as the buffer length to
optimize.
However, it can happen that there are alternatives calls like this one:
alternative("mfence; lfence", "", ALT_NOT(X86_FEATURE_APIC_MSRS_FENCE));
where the replacement length is 0. And using repl_len is wrong because
apply_alternatives() expands the buffer size to the length of the source
insn that is being patched, by padding it with one-byte NOPs:
for (; insn_buff_sz < a->instrlen; insn_buff_sz++)
insn_buff[insn_buff_sz] = 0x90;
Long story short: pass the length of the original instruction(s) as the
length of the temporary buffer which to optimize.
Result:
SMP alternatives: feat: 11*32+27, old: (lapic_next_deadline+0x9/0x50 (ffffffff81061829) len: 6), repl: (ffffffff89b1cc60, len: 0) flags: 0x1
SMP alternatives: ffffffff81061829: old_insn: 0f ae f0 0f ae e8
SMP alternatives: ffffffff81061829: final_insn: 90 90 90 90 90 90
=>
SMP alternatives: feat: 11*32+27, old: (lapic_next_deadline+0x9/0x50 (ffffffff81061839) len: 6), repl: (ffffffff89b1cc60, len: 0) flags: 0x1
SMP alternatives: ffffffff81061839: [0:6) optimized NOPs: 66 0f 1f 44 00 00
SMP alternatives: ffffffff81061839: old_insn: 0f ae f0 0f ae e8
SMP alternatives: ffffffff81061839: final_insn: 66 0f 1f 44 00 00
Fixes: da8f9cf7e721 ("x86/alternatives: Get rid of __optimize_nops()")
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240515104804.32004-1-bp@kernel.org
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
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https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.