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SubjectRe: [PATCHSET 00/13] tracing/uprobes: Add support for more fetch methods (v6)
Date
On Sat, 2 Nov 2013 16:54:58 +0100, Oleg Nesterov wrote:
> 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.

No no, this was not discussed enough. And I really appreciate your
thorough review! :)

>
> 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.

Hmm.. yes, I think this is not needed. I guess it should lookup a
proper vma in current->mm with mmap_sem read-locked.

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

Right. This is also an unnecessary restriction. We should be able to
access data in other binary.

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

I can't understand why.. The bss region should also be in a same vma of
normal data, no?

>
> - 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.

Argh.. This is a problem.

I thought the gcc somehow aligns data to next page boundary. But if
it's not the case, we need to recognize which is the proper one..

Simply preferring a writable vma to a read-only vma is what's came to my
head now. Do you have an idea?

>
> -------------------------------------------------------------------------------
> 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?

This can only work with the probes fetching data from the executable,
right? But as I said it should support any other binaries too.

What about this?


static void __user *get_user_vaddr(unsigned long addr, struct trace_uprobe *tu)
{
unsigned long pgoff = addr >> PAGE_SHIFT;
struct vm_area_struct *vma, *orig_vma = NULL;
unsigned long vaddr = 0;

if (tu == NULL) {
/* A NULL tu means that we already got the vaddr */
return (void __force __user *) addr;
}

down_read(&current->mm->mmap_sem);

vma = current->mm->mmap;
do {
if (!vma->vm_file || vma->vm_file->f_inode != tu->inode) {
/*
* We found read-only mapping for this inode.
* (provided that all mappings for this inode
* have consecutive addresses)
*/
if (orig_vma)
break;
continue;
}

if (vma->vm_pgoff > pgoff ||
(vma->vm_pgoff + vma_pages(vma) <= pgoff))
continue;

orig_vma = vma;

/*
* We prefer writable mapping over read-only since
* data is usually in read/write memory region. But
* in case of read-only data, it only can be found in
* read-only mapping so we save orig_vma and check
* whether it also has writable mapping.
*/
if (vma->vm_flags & VM_WRITE)
break;
} while ((vma = vma->vm_next) != NULL);

if (orig_vma)
vaddr = offset_to_vaddr(orig_vma, addr);

up_read(&current->mm->mmap_sem);

return (void __force __user *) vaddr;
}


>
> -------------------------------------------------------------------------------
> 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.

Discussion and feedbacks are always more than welcome. :)

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

For per-process uprobe, it might be useful.

>
> but lets ignore this for now.

Okay. Let's discuss again after solving current issues.

Thanks,
Namhyung


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