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Re: Tests requiring MD hooks
On Jul 11, 2011, at 11:59 PM, David Laight wrote:
> On Sun, Jul 10, 2011 at 09:38:13PM -0700, Matt Thomas wrote:
>> On Jul 10, 2011, at 7:58 PM, Valeriy E. Ushakov wrote:
>>>> That seems to be a bit tricky to write in a portable way: testing --x
>>>> right in memory pages requires to put/copy some payload in a page and
>>>> call it.
>>> dlopen something, find a function with dlsym, disable exec on that
>>> page and call the function. Catch SIGSEGV, check siginfo. mprotect
>>> it back and call the function again.
>> doesn't work when the target uses function descriptors.
> I'm confused (not difficult these days!).
> dlsym() returns 'void *', there is no separate dlsym_function_ptr() that
> can return a 'fat' function pointer, or is the symbol attached to
> something more akin to an x86 'call gate descriptor' - which is used
> as the target of the call instruction?
On hppa and powerpc64, dlsym returns a pointer to function descriptor. On
powerpc64, that contains a pointer to the function, a pointer to its TOC (GOT),
and a pointer to an environment address (not used in C).
mflr %r0 get return address
ld %r11,16(%r3) loads environment addressj
std %r0,16(%r1) save return address
stdu %r1,-112(%r1) allocate stack frame
ld %r0,0(%r3) loads function address
mtctr %r0 save to indirect register
std %r2,40(%r1) save our toc address
ld %r2,8(%r3) loads function's TOC
bctrl indirect call
ld 2,40(1) restore our toc address
addi %r1,%r1,112 free stack frame
ld %r0,16(%r1) load return address
mtlr %r0 save to indirect register
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