Re: [Qemu-devel] [PATCH] qemu_opts_append: Play nicely with QemuOptsList's head

[Chun Yan Liu]

>>> On 6/25/2014 at 04:38 PM, in message
<address@hidden>,
Michal Privoznik <address@hidden> wrote: 
> When running a libvirt test suite I've noticed the qemu-img is 
> crashing occasionally. Tracing the problem down led me to the 
> following valgrind output: 
>  
> qemu.git $ valgrind -q ./qemu-img create -f qed  
> -obacking_file=/dev/null,backing_fmt=raw qed 
> ==14881== Invalid write of size 8 
> ==14881==    at 0x1D263F: qemu_opts_create (qemu-option.c:692) 
> ==14881==    by 0x130782: bdrv_img_create (block.c:5531) 
> ==14881==    by 0x118DE0: img_create (qemu-img.c:462) 
> ==14881==    by 0x11E7E4: main (qemu-img.c:2830) 
> ==14881==  Address 0x11fedd38 is 24 bytes inside a block of size 232 free'd 
> ==14881==    at 0x4C2CA5E: realloc (in  
> /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so) 
> ==14881==    by 0x592D35E: g_realloc (in /usr/lib64/libglib-2.0.so.0.3800.2) 
> ==14881==    by 0x1D38D8: qemu_opts_append (qemu-option.c:1129) 
> ==14881==    by 0x13075E: bdrv_img_create (block.c:5528) 
> ==14881==    by 0x118DE0: img_create (qemu-img.c:462) 
> ==14881==    by 0x11E7E4: main (qemu-img.c:2830) 
> ==14881== 
> Formatting 'qed', fmt=qed size=0 backing_file='/dev/null' backing_fmt='raw'  
> cluster_size=65536 
> ==14881== Invalid write of size 8 
> ==14881==    at 0x1D28BE: qemu_opts_del (qemu-option.c:750) 
> ==14881==    by 0x130BF3: bdrv_img_create (block.c:5638) 
> ==14881==    by 0x118DE0: img_create (qemu-img.c:462) 
> ==14881==    by 0x11E7E4: main (qemu-img.c:2830) 
> ==14881==  Address 0x11fedd38 is 24 bytes inside a block of size 232 free'd 
> ==14881==    at 0x4C2CA5E: realloc (in  
> /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so) 
> ==14881==    by 0x592D35E: g_realloc (in /usr/lib64/libglib-2.0.so.0.3800.2) 
> ==14881==    by 0x1D38D8: qemu_opts_append (qemu-option.c:1129) 
> ==14881==    by 0x13075E: bdrv_img_create (block.c:5528) 
> ==14881==    by 0x118DE0: img_create (qemu-img.c:462) 
> ==14881==    by 0x11E7E4: main (qemu-img.c:2830) 
> ==14881== 
>  
> The problem is apparently in the qemu_opts_append(). Well, if it 
> gets called twice or more. On the first call, when @dst is NULL 
> some initialization is done during which @dst->head list gets 
> initialized. The list is initialized in a way, so that the list 
> tail points at the list head. However, the next time 
> qemu_opts_append() is called for new options to be added, 
> g_realloc() may move @dst at new address making the old list tail 
> point at invalid address. If that's the case we must update the 
> list pointers. 
>  
> Signed-off-by: Michal Privoznik <address@hidden> 
> --- 
>  util/qemu-option.c | 11 ++++++++++- 
>  1 file changed, 10 insertions(+), 1 deletion(-) 
>  
> diff --git a/util/qemu-option.c b/util/qemu-option.c 
> index 43de3ad..6ad2cf2 100644 
> --- a/util/qemu-option.c 
> +++ b/util/qemu-option.c 
> @@ -1111,6 +1111,7 @@ QemuOptsList *qemu_opts_append(QemuOptsList *dst, 
>      size_t num_opts, num_dst_opts; 
>      QemuOptDesc *desc; 
>      bool need_init = false; 
> +    bool need_head_update; 
>   
>      if (!list) { 
>          return dst; 
> @@ -1121,6 +1122,12 @@ QemuOptsList *qemu_opts_append(QemuOptsList *dst, 
>       */ 
>      if (!dst) { 
>          need_init = true; 
> +        need_head_update = true; 

Thanks very much for correction. But I think putting need_head_update here
is a little odd. How about delete this line and ... [1] 

> +    } else { 
> +        /* Moreover, even if dst is not NULL, the realloc may move it at a 
> +         * different address in which case we may get a stale tail pointer 
> +         * in dst->head. */ 

Only when dst->head is empty, we need to reinit, if dst->head->tqh_last points
to other places already,  we should not reinit. How about make it more clear?
e.g.
If dst->head is empty, then (dst->head)->tqh_last = &(dst->head)->tqh_first; 
after
g_realloc, (dst->head)->tqh_last may point to stail pointer, in this case, need 
to
reinit dst->head.

> +        need_head_update = QTAILQ_EMPTY(&dst->head); 
>      } 
>   
>      num_opts = count_opts_list(dst); 
> @@ -1131,9 +1138,11 @@ QemuOptsList *qemu_opts_append(QemuOptsList *dst, 
>      if (need_init) { 
>          dst->name = NULL; 
>          dst->implied_opt_name = NULL; 
> -        QTAILQ_INIT(&dst->head); 

[1] ... this line? (And initialize need_head_update = false.)

Thanks,
Chunyan

>          dst->merge_lists = false; 
>      } 
> +    if (need_head_update) { 
> +        QTAILQ_INIT(&dst->head); 
> +    } 
>      dst->desc[num_dst_opts].name = NULL; 
>   
>      /* append list->desc to dst->desc */ 
> --  
> 1.8.5.5 
>  
>  
>  
>