forked from Minki/linux
6b32c126d3
The alt_max_short() macro in asm/alternative.h does not work as
intended, leading to nasty bugs. E.g. alt_max_short("1", "3")
evaluates to 3, but alt_max_short("3", "1") evaluates to 1 -- not
exactly the maximum of 1 and 3.
In fact, I had to learn it the hard way by crashing my kernel in not
so funny ways by attempting to make use of the ALTENATIVE_2 macro
with alternatives where the first one was larger than the second
one.
According to [1] and commit dbe4058a6a ("x86/alternatives: Fix
ALTERNATIVE_2 padding generation properly") the right handed side
should read "-(-(a < b))" not "-(-(a - b))". Fix that, to make the
macro work as intended.
While at it, fix up the comments regarding the additional "-", too.
It's not about gas' usage of s32 but brain dead logic of having a
"true" value of -1 for the < operator ... *sigh*
Btw., the one in asm/alternative-asm.h is correct. And, apparently,
all current users of ALTERNATIVE_2() pass same sized alternatives,
avoiding to hit the bug.
[1] http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax
Reviewed-and-tested-by: Borislav Petkov <bp@suse.de>
Fixes: dbe4058a6a ("x86/alternatives: Fix ALTERNATIVE_2 padding generation properly")
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/1507228213-13095-1-git-send-email-minipli@googlemail.com
103 lines
2.4 KiB
C
103 lines
2.4 KiB
C
#ifndef _ASM_X86_ALTERNATIVE_ASM_H
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#define _ASM_X86_ALTERNATIVE_ASM_H
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#ifdef __ASSEMBLY__
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#include <asm/asm.h>
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#ifdef CONFIG_SMP
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.macro LOCK_PREFIX
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672: lock
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.pushsection .smp_locks,"a"
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.balign 4
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.long 672b - .
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.popsection
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.endm
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#else
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.macro LOCK_PREFIX
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.endm
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#endif
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/*
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* Issue one struct alt_instr descriptor entry (need to put it into
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* the section .altinstructions, see below). This entry contains
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* enough information for the alternatives patching code to patch an
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* instruction. See apply_alternatives().
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*/
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.macro altinstruction_entry orig alt feature orig_len alt_len pad_len
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.long \orig - .
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.long \alt - .
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.word \feature
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.byte \orig_len
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.byte \alt_len
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.byte \pad_len
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.endm
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/*
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* Define an alternative between two instructions. If @feature is
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* present, early code in apply_alternatives() replaces @oldinstr with
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* @newinstr. ".skip" directive takes care of proper instruction padding
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* in case @newinstr is longer than @oldinstr.
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*/
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.macro ALTERNATIVE oldinstr, newinstr, feature
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140:
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\oldinstr
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141:
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.skip -(((144f-143f)-(141b-140b)) > 0) * ((144f-143f)-(141b-140b)),0x90
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142:
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.pushsection .altinstructions,"a"
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altinstruction_entry 140b,143f,\feature,142b-140b,144f-143f,142b-141b
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.popsection
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.pushsection .altinstr_replacement,"ax"
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143:
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\newinstr
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144:
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.popsection
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.endm
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#define old_len 141b-140b
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#define new_len1 144f-143f
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#define new_len2 145f-144f
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/*
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* gas compatible max based on the idea from:
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* http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax
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*
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* The additional "-" is needed because gas uses a "true" value of -1.
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*/
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#define alt_max_short(a, b) ((a) ^ (((a) ^ (b)) & -(-((a) < (b)))))
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/*
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* Same as ALTERNATIVE macro above but for two alternatives. If CPU
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* has @feature1, it replaces @oldinstr with @newinstr1. If CPU has
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* @feature2, it replaces @oldinstr with @feature2.
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*/
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.macro ALTERNATIVE_2 oldinstr, newinstr1, feature1, newinstr2, feature2
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140:
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\oldinstr
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141:
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.skip -((alt_max_short(new_len1, new_len2) - (old_len)) > 0) * \
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(alt_max_short(new_len1, new_len2) - (old_len)),0x90
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142:
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.pushsection .altinstructions,"a"
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altinstruction_entry 140b,143f,\feature1,142b-140b,144f-143f,142b-141b
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altinstruction_entry 140b,144f,\feature2,142b-140b,145f-144f,142b-141b
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.popsection
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.pushsection .altinstr_replacement,"ax"
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143:
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\newinstr1
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144:
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\newinstr2
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145:
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.popsection
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.endm
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#endif /* __ASSEMBLY__ */
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#endif /* _ASM_X86_ALTERNATIVE_ASM_H */
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