Re: [DynInst_API:] image::findMain fails for -pie

[Adrian M Negreanu <groleo@xxxxxxxxx>]

On Fri, Nov 21, 2014 at 7:51 PM, Bill Williams <bill@xxxxxxxxxxx> wrote:

On 11/21/2014 11:36 AM, Josh Stone wrote:

On 11/21/2014 01:02 AM, Adrian M Negreanu wrote:

I tried to instrument a strip-ed chrome, as a workaround the
processCreate issue(s) , only to find another problem, this time in
image::findMain()

As a workaround, since you're the one stripping it, you can try
stripping slightly less. Just removing debuginfo with "strip -g" might
be enough to improve your processCreate issues, or use "strip -K main"
to at least keep main in the symbol table, making findMain unnecessary.

findMain() assumes the entry point address to be an immediate operand:
 mov  $0x40051b,%rdi
 callq 0x7ffff7fef3f0

For chrome, I found this instead
 lea -0x34f7(%rip),%rdi
 callq 26f20 <__libc_start_main@plt>

Josh found the same issue with a strip-ed stap, or as he put it
 "/usr/bin/stap is PIE, and I removed the debuginfo package"

Any suggestions for this one ?

I think ideally this should find the call in question, then use the
dataflowAPI to slice back and find the value of %rdi at this point. I
hope that's smart enough to evaluate things like -0x34f7(%rip), but I
haven't played with it much. PPC does something similar for r8.

On 32-bit x86, we'll want the last value on the stack instead.

The findMain logic here is sound, but I want to point out that stripping chrome is not actually going to help.

So it's sound to expect an immediate value for -pie binaries ? This eventually returns NULL, and
in that case the boostrap fails.
The strip-ing indeed, it doesn't fix the "processCreate taking too long", it only makes a bit bearable.

Â

Assuming that you don't need the preFork callback, chopping out the preFork instrumentation should be easy enough; I can send out a patch (probably late this afternoon or Monday) that implements the change I discussed in my most recent email if you want a slightly more polished version that doesn't break preFork wholesale.

Cool. I'll wait for the patch then.