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

[Denys Vlasenko <dvlasenk@redhat.com>]

On 04/28/2014 07:34 PM, Oleg Nesterov wrote:
> Thanks...
> 
> Again, the change in riprel_analyze() needs the review from someone
> who understands the instruction decoding/encoding.
> 
> On 04/28, Denys Vlasenko wrote:
>>
>> Otherwise, instructions such as cmpxchg and div will be mishandled.
> 
> 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:

# 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;
}