From: Matthew Fernandez <matthew.fernandez@gmail.com>
Date: Sun, 7 Aug 2022 02:52:18 +0000 (-0700)
Subject: cgraph agxbput_n: take an early exit when input string is zero
X-Git-Tag: 5.0.1~16^2~2
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=931977254bcc4947c14b726c107aa91b6e2ae3f5;p=graphviz

cgraph agxbput_n: take an early exit when input string is zero

An upcoming change makes it possible to enter this function with `xb->ptr` as
`NULL`. It looks as if there is nothing wrong with this; a zero-sized string
fits in any agxbuf, including an unallocated one. However UBSan educates us that
calling `memcpy` with a `NULL` destination pointer is undefined behavior, even
when the passed size is 0. So this change avoids reaching `memcpy` when we know
the function will be a no-op.

Note that this is sort of fixing a latent bug, in that an agxbuf user could
already cause the scenario described above by `agxbdisown`ing an agxbuf and then
calling `agxbput_n` on it with size 0. However no existing callers do this.
---

diff --git a/lib/cgraph/agxbuf.h b/lib/cgraph/agxbuf.h
index 12bf6c3f6..7412114bd 100644
--- a/lib/cgraph/agxbuf.h
+++ b/lib/cgraph/agxbuf.h
@@ -150,6 +150,9 @@ static inline PRINTF_LIKE(2, 3) int agxbprint(agxbuf *xb, const char *fmt,
  * Append string s of length ssz into xb
  */
 static inline size_t agxbput_n(agxbuf *xb, const char *s, size_t ssz) {
+  if (ssz == 0) {
+    return 0;
+  }
   if (xb->ptr + ssz > xb->eptr)
     agxbmore(xb, ssz);
   memcpy(xb->ptr, s, ssz);