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SubjectRe: [PATCHSET 00/13] tracing/uprobes: Add support for more fetch methods (v6)
Hello,

Let me first apologize again if this was already discussed. And I also
need to mention that I know almost nothing about elf/randomization/etc.

However,

On 10/29, Namhyung Kim wrote:
>
> # nm foo | grep -e glob$ -e str -e foo
> 00000000006008bc D foo
> 00000000006008a8 D glob
> 00000000006008ac D str
>
> # perf probe -x /home/namhyung/tmp/foo -a 'foo=main+0x13 glob=@0x8a8:s32 \

This does not look right to me.

- get_user_vaddr() is costly, it does vma_interval_tree_foreach() under
->i_mmap_mutex.

- this only allows to read the data from the same binary.

- in particular, you can't read the data from bss

- get_user_vaddr() looks simply wrong. I blindly applied the whole series
and did the test to ensure.

Test-case:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

unsigned int global = 0x1234;

void func(void)
{
}

int main(void)
{
char cmd[64];

global = 0x4321;
func();

printf("addr = %p\n", &global);

sprintf(cmd, "cat /proc/%d/maps", getpid());
system(cmd);

return 0;
}

# nm foo | grep -w global
0000000000600a04 D global

# perf probe -x ./foo -a "func var=@0xa04:u32"
# perf record -e probe_foo:func ./foo
addr = 0x600a04
00400000-00401000 r-xp 00000000 fe:01 20958 /root/foo
00600000-00601000 rw-p 00000000 fe:01 20958 /root/foo
...

# perf script | tail -1
foo 555 [000] 1302.345642: probe_foo:func: (40059c) var=1234

Note that it reports "1234", not "4321". This is because
get_user_vaddr() finds another (1st) read-only mapping, and
prints the initial value of "global".

IOW, it reads the memory from 0x400a04, not from 0x600a04.

-------------------------------------------------------------------------------
Can't we simply implement get_user_vaddr() as

static void __user *get_user_vaddr(unsigned long addr, struct trace_uprobe *tu)
{
void __user *vaddr = (void __force __user *)addr;

/* A NULL tu means that we already got the vaddr */
if (tu)
vaddr += (current->mm->start_data & PAGE_MASK);

return vaddr;
}

?

I did this change, and now the test-case above works. And it also works
with "cc -pie -fPIC",

# nm foo | grep -w global
0000000000200c9c D global

# perf probe -x ./foo -a "func var=@0xc9c:u32"
# perf record -e probe_foo:func ./foo
...
# perf script | tail -1
foo 576 [001] 475.519940: probe_foo:func: (7ffe95ca3814) var=4321

What do you think?

-------------------------------------------------------------------------------
Note:
- I think that /* A NULL tu means that we already got the vaddr */
needs more discussion... IOW, I am not sure about 11/13.

- Perhaps it also makes sense to allow to pass the absolute address
(iow, += start_data should be conditional)

but lets ignore this for now.

Oleg.



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