Re: [RFC PATCH] linux-user/mmap: Return EFAULT for invalid addresses

[Laurent Vivier <laurent@vivier.eu>]

Le 08/01/2021 à 18:46, Richard Purdie a écrit :
> When using qemu-i386 to run gobject introspection parts of a webkitgtk 
> build using musl as libc on a 64 bit host, it sits in an infinite loop 
> of mremap calls of ever decreasing/increasing addresses.
> 
> I suspect something in the musl memory allocation code loops indefinitely
> if it only sees ENOMEM and only exits when it hits EFAULT.
> 
> According to the docs, trying to mremap outside the address space
> can/should return EFAULT and changing this allows the build to succeed.
> 
> There was previous discussion of this as it used to work before qemu 2.11
> and we've carried hacks to work around it since, this appears to be a
> better fix of the real issue?
> 
> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org
> 
> Index: qemu-5.2.0/linux-user/mmap.c
> ===================================================================
> --- qemu-5.2.0.orig/linux-user/mmap.c
> +++ qemu-5.2.0/linux-user/mmap.c
> @@ -727,7 +727,7 @@ abi_long target_mremap(abi_ulong old_add
>           !guest_range_valid(new_addr, new_size)) ||
>          ((flags & MREMAP_MAYMOVE) == 0 &&
>           !guest_range_valid(old_addr, new_size))) {
> -        errno = ENOMEM;
> +        errno = EFAULT;
>          return -1;
>      }
>  
> 
> 

I agree with that, the ENOMEM is returned when there is not enough virtual memory (the
mmap_find_vma() case).

According to the manpage, EFAULT is returned when old_addr and old_addr + old_size is an invalid
address space.

So:

    if (!guest_range_valid(old_addr, old_size)) {
        errno = EFAULT;
        return -1;
    }

But in the case of new_size and new_addr, it seems the good value to use is EINVAL.

So:

   if (((flags & MREMAP_FIXED) && !guest_range_valid(new_addr, new_size)) ||
       ((flags & MREMAP_MAYMOVE) == 0 && !guest_range_valid(old_addr, new_size))) {
        errno = EINVAL;
        return -1;
    }

Did you try that?

Thanks,
Laurent