Seg fault occurs when running statically compiled binary from kernel using call_usermodehelper
Russell King - ARM Linux
linux at arm.linux.org.uk
Wed Jul 10 14:52:00 EDT 2013
On Wed, Jul 10, 2013 at 05:34:11PM +0100, Will Deacon wrote:
> Ok, I've finally got to the bottom of this, but I'm not sure on the best way
> to fix it.
I don't think you have! You need to look back at the older ARM kernels
to really get to the bottom of this...
> The issue is that libc expects r0 to contain a function pointer
> to be invoked at exit (rtld_fini), to clean up after a dynamic linker. If
> this pointer is NULL, then it is ignored. We actually zero this pointer in
> our ELF_PLAT_INIT macro.
> At the same time, we have this strange code called next from the ARM ELF
> regs->ARM_r2 = stack; /* r2 (envp) */ \
> regs->ARM_r1 = stack; /* r1 (argv) */ \
> regs->ARM_r0 = stack; /* r0 (argc) */ \
> which puts argc into r0.
You're sort of right. It dates from the days when we had a.out binaries,
those required argc, argv and envp in r0/r1/r2 - and ARM kernels carried
this hack in binfmt_aout.c to make it work in conjunction with the above:
static int load_aout_binary(struct linux_binprm * bprm)
start_thread(regs, ex.a_entry, current->mm->start_stack);
ELF, on the other hand, never had that hack - ELF has always been zero
in r0, and it's always retrieved the argc/argv/envp off the stack.
As the above hack got dropped from the kernel (I don't think it ever made
it into mainline), I think we should be safe getting rid of this
initialization of regs->ARM_r0 to r2, leaving them all as zeros.
We should probably also remove the selection of HAVE_AOUT from
arch/arm/Kconfig too as this definitely won't work with any recent
kernel (certainly not without binfmt_aout.c hacked in the above way.)
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