Re: [PATCH] virtio-gpu-virgl: fix error handling in virgl_cmd_resource_create_blob

[Markus Armbruster <armbru@redhat.com>]

Honglei Huang <honghuan@amd.com> writes:

> The error handling logic was incorrect in virgl_cmd_resource_create_blob.
> virtio_gpu_create_mapping_iov() returns 0 on success and non-zero on 
> failure, but the code was checking whether to set the error response.
>
> The fix changes the condition from 'if (!ret)' to 'if (ret != 0)' to
> properly handle the return value, consistent with other usage patterns
> in the same codebase (see virtio-gpu.c:932 and virtio-gpu.c:354).
>
> Signed-off-by: Honglei Huang <honghuan@amd.com>
> ---
>  hw/display/virtio-gpu-virgl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/hw/display/virtio-gpu-virgl.c b/hw/display/virtio-gpu-virgl.c
> index 94ddc01f91..e60e1059df 100644
> --- a/hw/display/virtio-gpu-virgl.c
> +++ b/hw/display/virtio-gpu-virgl.c
> @@ -701,7 +701,7 @@ static void virgl_cmd_resource_create_blob(VirtIOGPU *g,
>          ret = virtio_gpu_create_mapping_iov(g, cblob.nr_entries, sizeof(cblob),
>                                              cmd, &res->base.addrs,
>                                              &res->base.iov, &res->base.iov_cnt);
> -        if (!ret) {
> +        if (ret != 0) {

I recommend

           if (ret < 0) {

Why?

When a function returns true on success, false on error, we check for
error with

           if (!fn(...)) {

Same for functions returning a non-null pointer on success, null on
error.

When a function returns non-negative integer on success, negative
integer on error, we use

           if (fn(...) < 0) {

When a function returns zero on success, negative on error, both

           if (fn(...) < 0) {

and

           if (fn(...)) {

work.  I strongly prefer the former.  Why?

If fn() returns an integer, fn(...) < 0 is very likely correct (it's
incorrect only if fn() deviates from "return negative on error", which
is a bad idea).  If it returns a pointer or bool, fn(...) < 0 won't
compile.

If fn() returns an integer, fn(...) or fn(...) != 0 are likely correct
(same argument).  If it doesn't, they are likely backwards.

Because of this, an error check fn(...) == 0 triggers my spider sense
when I read the code: I stop and look up fn(...) to verify the error
check is correct.

Please don't write code that makes me stop and look up things when I
read it :)

>              cmd->error = VIRTIO_GPU_RESP_ERR_UNSPEC;
>              return;
>          }