Re: [Xen-devel] [PATCH] libxc: check return values on mmap() and madvise() on xc_alloc_hypercall_buffer()

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-05-20 at 05:37 -0700, Luis R. Rodriguez wrote:
> From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>
> 
> On a Thinkpad T4440p with OpenSUSE tumbleweed with v3.15-rc4
> and today's latest xen tip from the git tree strace -f reveals
> we end up on a never ending wait shortly after
> 
> write(20, "backend/console/5\0", 18 <unfinished ...>
> 
> This is right before we just wait on the qemu process which we
> had mmap'd for. Without this you'll end up getting stuck on a
> loop if mmap() worked but madvise() did not. While at it I noticed
> even the mmap() error fail was not being checked, fix that too.
> 
> Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx>

Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx> and applied.

OOI why was madvise failing? (should be quite unusual I think?)

> ---
>  tools/libxc/xc_linux_osdep.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/libxc/xc_linux_osdep.c b/tools/libxc/xc_linux_osdep.c
> index 73860a2..86bff3e 100644
> --- a/tools/libxc/xc_linux_osdep.c
> +++ b/tools/libxc/xc_linux_osdep.c
> @@ -92,14 +92,32 @@ static void 
> *linux_privcmd_alloc_hypercall_buffer(xc_interface *xch, xc_osdep_ha
>  {
>      size_t size = npages * XC_PAGE_SIZE;
>      void *p;
> +    int rc, saved_errno;
>  
>      /* Address returned by mmap is page aligned. */
>      p = mmap(NULL, size, PROT_READ|PROT_WRITE, 
> MAP_PRIVATE|MAP_ANONYMOUS|MAP_LOCKED, -1, 0);
> +    if ( p == MAP_FAILED )
> +    {
> +        PERROR("xc_alloc_hypercall_buffer: mmap failed");
> +        return NULL;
> +    }
>  
>      /* Do not copy the VMA to child process on fork. Avoid the page being COW
>          on hypercall. */
> -    madvise(p, npages * XC_PAGE_SIZE, MADV_DONTFORK);
> +    rc = madvise(p, npages * XC_PAGE_SIZE, MADV_DONTFORK);
> +    if ( rc < 0 )
> +    {
> +        PERROR("xc_alloc_hypercall_buffer: madvise failed");
> +        goto out;
> +    }
> +
>      return p;
> +
> +out:
> +    saved_errno = errno;
> +    (void)munmap(p, size);
> +    errno = saved_errno;
> +    return NULL;
>  }
>  
>  static void linux_privcmd_free_hypercall_buffer(xc_interface *xch, 
> xc_osdep_handle h, void *ptr, int npages)



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