GDB might trash EAX when stepping into a PLT call

[dlangBugzillaToGithub]

Martin Nowak (@MartinNowak) reported this on 2013-05-29T19:16:53Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=10210

CC List

• Walter Bright (@WalterBright)

Description

This affects shared libraries on X86 32-bit. The linker stub doesn't restore the content of EAX. This may lead to corruption because dmd may pass an argument in EAX.

[dlangBugzillaToGithub]

code (@MartinNowak) commented on 2013-05-31T11:52:36Z

Until now I don't know any solution other than to not use EAX for parameter passing. Maybe ECX or EDX would work as alternative.

[dlangBugzillaToGithub]

bugzilla (@WalterBright) commented on 2013-06-06T14:10:46Z

An example would be nice, please!

[dlangBugzillaToGithub]

code (@MartinNowak) commented on 2013-06-06T15:14:37Z

cat > bug.d << CODE
import lib;

void main()
{
    int val;
    printf("%p\n", &val);
    deref(&val) == 0 || assert(0);
}
CODE

cat > lib.d << CODE
extern(C) int printf(const char*, ...);

int deref(int* p)
{
    printf("%p\n", p);
    return *p;
}
CODE

dmd -g -m32 -fPIC -shared lib.d -ofliblib.so
dmd -g -m32 -fPIC bug.d -L-L. -L-llib -L-rpath=.
./bug

----

Oh, the above code actually works. It's not the PLT stub but the attached debugger (gdb-7.5.1) that trashes EAX. It only happens when stepping into a function called via PLT and I can only reproduce with emacs-GUD, probably because of some automatic variable update.

It also means there is another reason why phobos unittests crash on X86 :(.