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Re: [PATCH] virtiofsd: Fix side-effect in assert()

From: Stefan Weil
Subject: Re: [PATCH] virtiofsd: Fix side-effect in assert()
Date: Fri, 9 Apr 2021 17:11:13 +0200
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.9.0

Am 09.04.21 um 12:06 schrieb Greg Kurz:

It is bad practice to put an expression with a side-effect in
assert() because the side-effect won't happen if the code is
compiled with -DNDEBUG.

Use an intermediate variable. Consolidate this in an macro to
have proper line numbers when the assertion is hit.

virtiofsd: ../../tools/virtiofsd/passthrough_ll.c:2797: lo_getxattr:
  Assertion `fchdir_res == 0' failed.

   2796          /* fchdir should not fail here */
=>2797          FCHDIR_NOFAIL(lo->proc_self_fd);
   2798          ret = getxattr(procname, name, value, size);
   2799          FCHDIR_NOFAIL(lo->root.fd);

Fixes: bdfd66788349 ("virtiofsd: Fix xattr operations")
Cc: misono.tomohiro@jp.fujitsu.com
Signed-off-by: Greg Kurz <groug@kaod.org>
  tools/virtiofsd/passthrough_ll.c | 21 +++++++++++++--------
  1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 1553d2ef454f..6592f96f685e 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -2723,6 +2723,11 @@ static int xattr_map_server(const struct lo_data
*lo, const char *server_name,
      return -ENODATA;
+#define FCHDIR_NOFAIL(fd) do { \
+        int fchdir_res = fchdir(fd);                   \
+        assert(fchdir_res == 0);                       \
+    } while (0)
  static void lo_getxattr(fuse_req_t req, fuse_ino_t ino, const char *in_name,

I am afraid that this will raise a compiler warning (or error with -Werror) when NDEBUG is defined because the variable is unused in that case ([-Wunused-variable]).

I suggest to use different implementations of the macro depending on NDEBUG.


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