Flash LAHF workaround

December 08, 2009

Flash uses an instruction not available on some CPUs, causing it to die with SIGILL ("Illegal instruction"). Some might say, "Ok, let's wait for Adobe to release a fix." But not Maks Verver, who wrote this bad ass patch:

/* Simple work-around for running the 64-bit Adobe Flash plug-in version 10
   on Athlon64 processors without support for the lahf instruction.

Compile with:
cc -fPIC -shared -nostdlib -lc -oflashplugin-lahf-fix.so flashplugin-lahf-fix.c
Then place the .so file in the plug-in directory (e.g. $HOME/.mozilla/plugins)
or use LD_PRELOAD to force Firefox to load the library.

   - Maks Verver <maksverver@geocities.com> July 2009 */

#define _GNU_SOURCE
#include <stdlib.h>
#include <signal.h>
#include <ucontext.h>

static void sig_handler(int signal, siginfo_t *info, void *context) {
        if (signal != SIGILL) return;
        if (*(char*)info->si_addr != (char)0x9f) abort();
        greg_t *regs = ((ucontext_t*)context)->uc_mcontext.gregs;
        ((char*)&regs[REG_RAX])[1] = ((char*)&regs[REG_EFL])[0];

static struct sigaction old_sa, new_sa = {
        .sa_flags     = SA_SIGINFO,
        .sa_sigaction = &sig_handler };

int _init() { sigaction(SIGILL, &new_sa, &old_sa); return 0; }
int _fini() { sigaction(SIGILL, &old_sa, &new_sa); return 0; }

It catches the signal, inspects the registers, and by hand updates the state and advances the instruction pointer.