linux_dsm_epyc7002/arch/x86/include/asm/alternative-asm.h
Borislav Petkov dbe4058a6a x86/alternatives: Fix ALTERNATIVE_2 padding generation properly
Quentin caught a corner case with the generation of instruction
padding in the ALTERNATIVE_2 macro: if len(orig_insn) <
len(alt1) < len(alt2), then not enough padding gets added and
that is not good(tm) as we could overwrite the beginning of the
next instruction.

Luckily, at the time of this writing, we don't have
ALTERNATIVE_2() invocations which have that problem and even if
we did, a simple fix would be to prepend the instructions with
enough prefixes so that that corner case doesn't happen.

However, best it would be if we fixed it properly. See below for
a simple, abstracted example of what we're doing.

So what we ended up doing is, we compute the

	max(len(alt1), len(alt2)) - len(orig_insn)

and feed that value to the .skip gas directive. The max() cannot
have conditionals due to gas limitations, thus the fancy integer
math.

With this patch, all ALTERNATIVE_2 sites get padded correctly;
generating obscure test cases pass too:

  #define alt_max_short(a, b)    ((a) ^ (((a) ^ (b)) & -(-((a) < (b)))))

  #define gen_skip(orig, alt1, alt2, marker)	\
  	.skip -((alt_max_short(alt1, alt2) - (orig)) > 0) * \
  		(alt_max_short(alt1, alt2) - (orig)),marker

  	.pushsection .text, "ax"
  .globl main
  main:
  	gen_skip(1, 2, 4, 0x09)
  	gen_skip(4, 1, 2, 0x10)
  	...
  	.popsection

Thanks to Quentin for catching it and double-checking the fix!

Reported-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150404133443.GE21152@pd.tnic
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2015-04-04 15:58:23 +02:00

83 lines
1.6 KiB
C

#ifndef _ASM_X86_ALTERNATIVE_ASM_H
#define _ASM_X86_ALTERNATIVE_ASM_H
#ifdef __ASSEMBLY__
#include <asm/asm.h>
#ifdef CONFIG_SMP
.macro LOCK_PREFIX
672: lock
.pushsection .smp_locks,"a"
.balign 4
.long 672b - .
.popsection
.endm
#else
.macro LOCK_PREFIX
.endm
#endif
.macro altinstruction_entry orig alt feature orig_len alt_len pad_len
.long \orig - .
.long \alt - .
.word \feature
.byte \orig_len
.byte \alt_len
.byte \pad_len
.endm
.macro ALTERNATIVE oldinstr, newinstr, feature
140:
\oldinstr
141:
.skip -(((144f-143f)-(141b-140b)) > 0) * ((144f-143f)-(141b-140b)),0x90
142:
.pushsection .altinstructions,"a"
altinstruction_entry 140b,143f,\feature,142b-140b,144f-143f,142b-141b
.popsection
.pushsection .altinstr_replacement,"ax"
143:
\newinstr
144:
.popsection
.endm
#define old_len 141b-140b
#define new_len1 144f-143f
#define new_len2 145f-144f
/*
* max without conditionals. Idea adapted from:
* http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax
*/
#define alt_max_short(a, b) ((a) ^ (((a) ^ (b)) & -(-((a) < (b)))))
.macro ALTERNATIVE_2 oldinstr, newinstr1, feature1, newinstr2, feature2
140:
\oldinstr
141:
.skip -((alt_max_short(new_len1, new_len2) - (old_len)) > 0) * \
(alt_max_short(new_len1, new_len2) - (old_len)),0x90
142:
.pushsection .altinstructions,"a"
altinstruction_entry 140b,143f,\feature1,142b-140b,144f-143f,142b-141b
altinstruction_entry 140b,144f,\feature2,142b-140b,145f-144f,142b-141b
.popsection
.pushsection .altinstr_replacement,"ax"
143:
\newinstr1
144:
\newinstr2
145:
.popsection
.endm
#endif /* __ASSEMBLY__ */
#endif /* _ASM_X86_ALTERNATIVE_ASM_H */