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

[Philippe Mathieu-Daudé]

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.merw...@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 <phi...@linaro.org>