[Libguestfs] [PATCH] lib/errors.c: Fix usage of strerror_r

[Richard W.M. Jones]

  $ ./run guestfish -vx add-drive foo "readonly:true"
  libguestfs: trace: set_pgroup true
  libguestfs: trace: set_pgroup = 0
  libguestfs: trace: add_drive "foo" "readonly:true"
  libguestfs: error: foo:
  libguestfs: trace: add_drive = -1 (error)
  libguestfs: trace: close
  libguestfs: closing guestfs handle 0x55c0bacf12a0 (state 0)

Note the "error: foo: " line.  After hexdumping this I found that it
is actually printing random rubbish after the colon.  It turns out
that strerror_r was not updating the buffer and so we were printing
random bytes from the stack.

strerror_r is hard to use and worse still there are two incompatible
variants in glibc.  Close reading of the GNU variant that we are using
shows that it doesn't always use the buffer passed in, which explains
what was happening here.

Try to fix usage of strerror_r and try to cope with the two variants
using _GNU_SOURCE.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2030396
---
 lib/errors.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/lib/errors.c b/lib/errors.c
index bd9699eeb1..d599a21879 100644
--- a/lib/errors.c
+++ b/lib/errors.c
@@ -322,6 +322,7 @@ guestfs_int_perrorf (guestfs_h *g, const char *fs, ...)
   const int errnum = errno;
   int err;
   char buf[256];
+  const char *errstr;
   struct error_data *error_data = get_error_data (g);
 
   va_start (args, fs);
@@ -330,11 +331,19 @@ guestfs_int_perrorf (guestfs_h *g, const char *fs, ...)
 
   if (err < 0) return;
 
-  ignore_value (strerror_r (errnum, buf, sizeof buf));
+#ifdef _GNU_SOURCE
+  /* GNU strerror_r */
+  errstr = strerror_r (errnum, buf, sizeof buf);
+#else
+  /* XSI-compliant strerror_r */
+  err = strerror_r (errnum, buf, sizeof buf);
+  if (err > 0) return;
+  errstr = buf;
+#endif
 
-  msg = safe_realloc (g, msg, strlen (msg) + 2 + strlen (buf) + 1);
+  msg = safe_realloc (g, msg, strlen (msg) + 2 + strlen (errstr) + 1);
   strcat (msg, ": ");
-  strcat (msg, buf);
+  strcat (msg, errstr);
 
   /* set_last_error first so that the callback can access the error
    * message and errno through the handle if it wishes.
-- 
2.32.0