The cmp utility has the questionable feature of using mmap when

comparing regular files.  Add a SIGSEGV handler to make its
behavior less surprising when a read error occurs.  The handler
does not attempt to distinguish errors from file truncation, but
anyone actively modifying a file while trying to compare it
shouldn't even expect something sane to happen.

PR:		45391
Reviewed by:	mike (mentor)

1 files changed, 21 insertions, 0 deletions

diff --git a/usr.bin/cmp/regular.c b/usr.bin/cmp/regular.c
index 7e38fcd..7a27f5e 100644
--- a/usr.bin/cmp/regular.c
+++ b/usr.bin/cmp/regular.c
@@ -45,7 +45,9 @@ __FBSDID("$FreeBSD$");
 #include <sys/stat.h>
 
 #include <err.h>
+#include <errno.h>
 #include <limits.h>
+#include <signal.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
@@ -54,6 +56,7 @@ __FBSDID("$FreeBSD$");
 #include "extern.h"
 
 static u_char *remmap(u_char *, int, off_t);
+static void segv_handler(int);
 #define MMAP_CHUNK (8*1024*1024)
 
 #define ROUNDPAGE(i) ((i) & ~pagemask)
@@ -67,6 +70,7 @@ c_regular(int fd1, const char *file1, off_t skip1, off_t len1,
 	int dfound;
 	off_t pagemask, off1, off2;
 	size_t pagesize;
+	struct sigaction act, oact;
 
 	if (skip1 > len1)
 		eofmsg(file1);
@@ -78,6 +82,12 @@ c_regular(int fd1, const char *file1, off_t skip1, off_t len1,
 	if (sflag && len1 != len2)
 		exit(DIFF_EXIT);
 
+	sigemptyset(&act.sa_mask);
+	act.sa_flags = SA_NODEFER;
+	act.sa_handler = segv_handler;
+	if (sigaction(SIGSEGV, &act, &oact))
+		err(ERR_EXIT, "sigaction()");
+
 	pagesize = getpagesize();
 	pagemask = (off_t)pagesize - 1;
 	off1 = ROUNDPAGE(skip1);
@@ -138,6 +148,9 @@ c_regular(int fd1, const char *file1, off_t skip1, off_t len1,
 	munmap(m1, MMAP_CHUNK);
 	munmap(m2, MMAP_CHUNK);
 
+	if (sigaction(SIGSEGV, &oact, NULL))
+		err(ERR_EXIT, "sigaction()");
+
 	if (len1 != len2)
 		eofmsg (len1 > len2 ? file2 : file1);
 	if (dfound)
@@ -155,3 +168,11 @@ remmap(u_char *mem, int fd, off_t offset)
 	madvise(mem, MMAP_CHUNK, MADV_SEQUENTIAL);
 	return (mem);
 }
+
+static void
+segv_handler(int sig) {
+	static const char msg[] = "cmp: Input/output error (caught SIGSEGV)\n";
+
+	write(STDERR_FILENO, msg, sizeof(msg));
+	_exit(EXIT_FAILURE);
+}