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

[Richard Purdie <richard.purdie@linuxfoundation.org>]

On Sat, 2021-02-13 at 18:40 +0100, Laurent Vivier wrote:
> 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?

Its taken me a short while to reproduce the test environment but I did
so and can confirm that using EINVAL works just as well as EFAULT in
the test case we have. The above would therefore seem to make sense to
me and would fix the case we found.

Cheers,

Richard