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Re: [PATCH] Omit frame pointer and fix %ebp by default on x86 (take3)


On Mon, 16 Aug 2004, Jeffrey A Law wrote:
> Do the new versions of oprofile need this data?  They have some
> limited capability to get backtraces when events occur and I thought
> it used the frame pointer to do that (otherwise we'd have to do
> something ugly like embed a dwarf2 frame unwinder into the kernel and
> run it when events occur -- not good).

I've checked and indeed the call-graph patch to "oprofile" requires a
frame-pointer to report the call-graph.  However, as pointed out in
that patch's documentation, this requires the Linux kernel to be rebuild
with CONFIG_FRAME_POINTER to allow it to profile the kernel, as most
distributions omit the frame poiner by default.


Taking a slightly different tack, preserving the frame pointer in
executables that don't need it is a potential security vulnerability.
The ability to walk back through stack frames inspecting PCs allows
a "code insertion/buffer overflow" attack to subvert even recent
position-independent-executable (PIE) security polices.  By knowing
how many frames deep an exploit is within a known executable, the
attacking code can unwind to a known reference point in both the
executable, libc and maybe even the kernel.  Once the exploit has
the relocated address of __libc_start_main in libc.so, its possible
to invoke system library functions without link-time support.

Omitting frame pointers makes this process significantly harder.

This only affects x86 applications built with GCC that aren't compiled
with -fomit-frame-pointer.  Software build with compilers that don't
reserve %ebp for a frame pointer, such as Microsoft's and Intel's,
should be unaffected.

Ok, its a long shot...

Roger
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