Re: [PATCH] hw/xen: Fix trace_xs_node_read() params

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 23/5/25 18:01, Liam Merwick wrote:
> When the '--enable-trace-backends=syslog' build option is configured,
> the following compilation error is encountered.
> 
> In file included from /usr/include/sys/syslog.h:207,
>                   from /usr/include/syslog.h:1,
>                   from ./trace/trace-hw_xen.h:224,
>                   from ../hw/xen/trace.h:1,
>                   from ../hw/xen/xen-bus-helper.c:13:
> In function ‘syslog’,
>      inlined from ‘_nocheck__trace_xs_node_read’ at ../hw/xen/trace-events:41:9,
>      inlined from ‘trace_xs_node_read’ at trace/trace-hw_xen.h:903:9,
>      inlined from ‘xs_node_read’ at ../hw/xen/xen-bus-helper.c:154:5:
> /usr/include/bits/syslog.h:45:3: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
>     45 |   __syslog_chk (__pri, __USE_FORTIFY_LEVEL - 1, __fmt, __va_arg_pack ());
>        |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Add a check that 'value' is not null before passing it to trace_xs_node_read().
> 
> Fixes: e6cdeee95990 ("hw/xen: Add xs_node_read() helper function")
> Signed-off-by: Liam Merwick <liam.merwick@oracle.com>
> ---
>   hw/xen/xen-bus-helper.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/xen/xen-bus-helper.c b/hw/xen/xen-bus-helper.c
> index 288fad422be3..1087a585cc71 100644
> --- a/hw/xen/xen-bus-helper.c
> +++ b/hw/xen/xen-bus-helper.c
> @@ -151,7 +151,7 @@ char *xs_node_read(struct qemu_xs_handle *h, xs_transaction_t tid,
>       va_end(ap);
>   
>       value = qemu_xen_xs_read(h, tid, path, len);
> -    trace_xs_node_read(path, value);
> +    trace_xs_node_read(path, value ? value : "<null>");
>       if (!value) {
>           error_setg_errno(errp, errno, "failed to read from '%s'", path);
>       }

Alternatively, since this is an error path:

-- >8 --
diff --git a/hw/xen/xen-bus-helper.c b/hw/xen/xen-bus-helper.c
index 288fad422be..1e49e60e147 100644
--- a/hw/xen/xen-bus-helper.c
+++ b/hw/xen/xen-bus-helper.c
@@ -143,7 +143,8 @@ char *xs_node_read(struct qemu_xs_handle *h, 
xs_transaction_t tid,
                     unsigned int *len, Error **errp,
                     const char *path_fmt, ...)
  {
-    char *path, *value;
+    g_autofree char *value;
+    char *path;
      va_list ap;

      va_start(ap, path_fmt);
@@ -151,12 +152,11 @@ char *xs_node_read(struct qemu_xs_handle *h, 
xs_transaction_t tid,
      va_end(ap);

      value = qemu_xen_xs_read(h, tid, path, len);
-    trace_xs_node_read(path, value);
      if (!value) {
          error_setg_errno(errp, errno, "failed to read from '%s'", path);
+        return NULL;
      }
-
-    g_free(path);
+    trace_xs_node_read(path, value);

      return value;
  }
---

But your patch isn't wrong, so:
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>