Re: [PATCH] linux-user: mremap fails with EFAULT if address range overlaps with stack guard

[Tobias Koch <tobias.koch@nonterra.com>]

Hi Laurent,

the code in musl libc probing the stack is in

    https://git.musl-libc.org/cgit/musl/plain/src/thread/pthread_getattr_np.c

The setup in elfload.c does work, but only when reserved_va is not set. In that case, any stack guard violation is
handled by the host kernel and thus results in the expected EFAULT.

However, in case of e.g. a 32bit target being emulated on a 64bit host, reserved_va is set and the current code in
mmap.c will only produce a more generic ENOMEM, deviating from the kernel's behavior.


On 5/7/20 5:35 PM, Laurent Vivier wrote:
> Le 05/03/2020 à 22:05, Tobias Koch a écrit :
>> If the address range starting at old_address overlaps with the stack guard it
>> is invalid and mremap must fail with EFAULT. The musl c library relies on this
>> behavior to detect the stack size, which it does by doing consecutive mremaps
>> until it hits the stack guard. Without this patch, software (such as the Ruby
>> interpreter) that calls pthread_getattr_np under musl will crash on 32 bit
>> targets emulated on a 64 bit host.
> Could you share some pointers to the code that is doing this?
>
> We have already this kind of code in linux-user/elfload.c,
> setup_arg_pages(): could you check why it doesn't work?
>
> Thanks,
> Laurent