Re: [Xen-devel] [PATCH] [v2] libxc: Replace alloca() with mmap() for pfn array sizes greater than a page in linux_privcmd_map_foreign_bulk()

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2012-04-20 at 19:38 +0100, Aravindh Puthiyaparambil wrote:
> When mapping in large amounts of pages (in the GB range) from a guest in to 
> Dom0 using xc_map_foreign_bulk(), a segfault occurs in the libxc client 
> application. This is because the pfn array in 
> linux_privcmd_map_foreign_bulk() is being allocated using alloca() and the 
> subsequent memcpy causes the stack to blow. This patch replaces the alloca() 
> with mmap() for pfn array sizes greater than a page.
> 
> Fix an error print with the correct function name.
> 
> Signed-off-by: Aravindh Puthiyaparambil <aravindh@xxxxxxxxxxxx>

Thanks. Did you rule out this bug at the  usage of alloca() in
linux_gnttab_grant_map? If not then I think that should be changed too.

> Acked-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
> 
> diff -r 7c777cb8f705 -r e62ab14d44af tools/libxc/xc_linux_osdep.c
> --- a/tools/libxc/xc_linux_osdep.c    Wed Apr 18 16:49:55 2012 +0100
> +++ b/tools/libxc/xc_linux_osdep.c    Fri Apr 20 11:36:02 2012 -0700
> @@ -39,6 +39,7 @@
>  #include "xenctrl.h"
>  #include "xenctrlosdep.h"
>  
> +#define ROUNDUP(_x,_w) (((unsigned long)(_x)+(1UL<<(_w))-1) & 
> ~((1UL<<(_w))-1))
>  #define ERROR(_m, _a...)  xc_osdep_log(xch,XTL_ERROR,XC_INTERNAL_ERROR,_m , 
> ## _a )
>  #define PERROR(_m, _a...) xc_osdep_log(xch,XTL_ERROR,XC_INTERNAL_ERROR,_m \
>                    " (%d = %s)", ## _a , errno, xc_strerror(xch, errno))
> @@ -258,7 +259,7 @@ static void *linux_privcmd_map_foreign_b
>                  fd, 0);
>      if ( addr == MAP_FAILED )
>      {
> -        PERROR("xc_map_foreign_batch: mmap failed");
> +        PERROR("xc_map_foreign_bulk: mmap failed");
>          return NULL;
>      }
>  
> @@ -286,7 +287,21 @@ static void *linux_privcmd_map_foreign_b
>           * IOCTL_PRIVCMD_MMAPBATCH.
>           */
>          privcmd_mmapbatch_t ioctlx;
> -        xen_pfn_t *pfn = alloca(num * sizeof(*pfn));
> +        xen_pfn_t *pfn;
> +        unsigned int pfn_arr_size = ROUNDUP((num * sizeof(*pfn)), 
> XC_PAGE_SHIFT);
> +
> +        if ( pfn_arr_size <= XC_PAGE_SIZE )
> +            pfn = alloca(num * sizeof(*pfn));
> +        else
> +        {
> +            pfn = mmap(NULL, pfn_arr_size, PROT_READ | PROT_WRITE,
> +                       MAP_PRIVATE | MAP_ANON | MAP_POPULATE, -1, 0);
> +            if ( pfn == MAP_FAILED )
> +            {
> +                PERROR("xc_map_foreign_bulk: mmap of pfn array failed");
> +                return NULL;
> +            }
> +        }
>  
>          memcpy(pfn, arr, num * sizeof(*arr));
>  
> @@ -328,6 +343,9 @@ static void *linux_privcmd_map_foreign_b
>              break;
>          }
>  
> +        if ( pfn_arr_size > XC_PAGE_SIZE )
> +            munmap(pfn, pfn_arr_size);
> +
>          if ( rc == -ENOENT && i == num )
>              rc = 0;
>          else if ( rc )



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