[PATCH] tail: avoid theoretically undefined behavior

[Jim Meyering]

Can anyone name a real system on which forming a pointer like this,
"buffer + (size_t)(-1)" actually provokes a trap or some other real problem?


>From 6e00315bf290310895036fce979a7e0210871b63 Mon Sep 17 00:00:00 2001
From: Jim Meyering <address@hidden>
Date: Wed, 28 Dec 2011 18:30:50 +0100
Subject: [PATCH] tail: avoid theoretically undefined behavior

* src/tail.c (start_lines): Do not form potentially-invalid address.
Use safe_read's return value as a pointer offset only after
ensuring that it is not SAFE_READ_ERROR (size_t)(-1).
Spotted by coverity.
Also, move declaration of "p" to be closer to first use.
---
 src/tail.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/tail.c b/src/tail.c
index 4581845..5d86da2 100644
--- a/src/tail.c
+++ b/src/tail.c
@@ -848,9 +848,7 @@ start_lines (const char *pretty_filename, int fd, uintmax_t 
n_lines,
   while (1)
     {
       char buffer[BUFSIZ];
-      char *p = buffer;
       size_t bytes_read = safe_read (fd, buffer, BUFSIZ);
-      char *buffer_end = buffer + bytes_read;
       if (bytes_read == 0) /* EOF */
         return -1;
       if (bytes_read == SAFE_READ_ERROR) /* error */
@@ -859,8 +857,11 @@ start_lines (const char *pretty_filename, int fd, 
uintmax_t n_lines,
           return 1;
         }

+      char *buffer_end = buffer + bytes_read;
+
       *read_pos += bytes_read;

+      char *p = buffer;
       while ((p = memchr (p, '\n', buffer_end - p)))
         {
           ++p;
--
1.7.8.1.391.g2c2ad