linux-kernel - Re: [PATCH] uprobes: use BX register for rip-relative fixups, not AX

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Date:	Mon, 28 Apr 2014 21:23:04 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Jim Keniston <jkenisto@...ibm.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] uprobes: use BX register for rip-relative fixups, not
	AX

On 04/28, Denys Vlasenko wrote:
>
> On 04/28/2014 07:34 PM, Oleg Nesterov wrote:
> >
> > It seems that you are right. But it would be really great if you also
> > provide the test-case which proves the fix ;)
>
> Working on a testcase for this. So far covered div (test1)
> and cmpxchg (test2).
>
> Reproduced failure on a fairly old 3.10.11 kernel:

Just in case, confirm. Reproduced on v3.14 + all recent uprobes changes.

Thanks.

> # gcc -Os -Wall test_riprel.c -o test_riprel
> # ./test_riprel
> test1: pass
> test2: pass
> # perf probe -x ./test_riprel probe1
> # perf record -e probe_test:probe1 ./test_riprel
> test1: FAIL
> test2: pass
> # perf probe -x ./test_riprel probe2
> # perf record -e probe_test:probe2 ./test_riprel
> test1: pass
> test2: FAIL
> 
> Source:
> 
> test_riprel.c
> ==================
> #include <stdio.h>
> 
> static const char *const fail_pass[] = { "FAIL", "pass" };
> 
> long two = 2;
> long test1()
> {
> 	long ax=0, dx=0;
> 	asm volatile("\n"
> "			xor	%%edx,%%edx\n"
> "			lea	2(%%edx),%%eax\n"
> // We divide 2 by 2. Result (in eax) should be 1:
> "	probe1:		.globl	probe1\n"
> "			divl	two(%%rip)\n"
> // If we have a bug (eax mangled on entry) the result will be 2,
> // because eax gets restored by probe machinery.
> 	: "=a" (ax), "=d" (dx) /*out*/
> 	: "0" (ax), "1" (dx) /*in*/
> 	: "memory" /*clobber*/
> 	);
> 	dprintf(2, "%s: %s\n", __func__, fail_pass[ax == 1]);
> 	return ax;
> }
> 
> long val2 = 0;
> long test2()
> {
> 	long old_val2 = val2;
> 	long ax=0, dx=0;
> 	asm volatile("\n"
> "			mov	val2,%%eax\n"     // eax := val2
> "			lea	1(%%eax),%%edx\n" // edx := eax+1
> // eax is equal to val2. cmpxchg should store edx to val2:
> "	probe2:		.globl  probe2\n"
> "			cmpxchg %%edx,val2(%%rip)\n"
> // If we have a bug (eax mangled on entry), val2 will stay unchanged
> 	: "=a" (ax), "=d" (dx) /*out*/
> 	: "0" (ax), "1" (dx) /*in*/
> 	: "memory" /*clobber*/
> 	);
> 	dprintf(2, "%s: %s\n", __func__, fail_pass[val2 == old_val2 + 1]);
> 	return ax == dx;
> }
> 
> int main()
> {
> 	test1();
> 	test2();
> 	return 0;
> }
> 

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