From abb3f965d13ea86c79b56497a9ff022c422840ff Mon Sep 17 00:00:00 2001
From: John Baldwin <jhb@FreeBSD.org>
Date: Wed, 5 Aug 2015 18:14:01 +0000
Subject: [PATCH] Rework get_string() to make it more robust when fetching
 strings of unknown length.  In particular, instead of blinding fetching 1k
 blocks, do an initial fetch up to the end of the current page followed by
 page-sized fetches up to the maximum size.  Previously if the 1k buffer
 crossed a page boundary and the second page was not valid, the entire
 operation would fail.

---
 usr.bin/truss/syscalls.c | 46 +++++++++++++++++++++++++---------------
 1 file changed, 29 insertions(+), 17 deletions(-)

diff --git a/usr.bin/truss/syscalls.c b/usr.bin/truss/syscalls.c
index aa4bac5e82a..b758ddbda22 100644
--- a/usr.bin/truss/syscalls.c
+++ b/usr.bin/truss/syscalls.c
@@ -539,44 +539,56 @@ get_struct(pid_t pid, void *offset, void *buf, int len)
 }
 
 #define	MAXSIZE		4096
-#define	BLOCKSIZE	1024
+
 /*
  * get_string
  * Copy a string from the process.  Note that it is
  * expected to be a C string, but if max is set, it will
  * only get that much.
  */
-
 static char *
-get_string(pid_t pid, void *offset, int max)
+get_string(pid_t pid, void *addr, int max)
 {
 	struct ptrace_io_desc iorequest;
-	char *buf;
-	int diff, i, size, totalsize;
+	char *buf, *nbuf;
+	size_t offset, size, totalsize;
 
-	diff = 0;
-	totalsize = size = max ? (max + 1) : BLOCKSIZE;
+	offset = 0;
+	if (max)
+		size = max + 1;
+	else {
+		/* Read up to the end of the current page. */
+		size = PAGE_SIZE - ((uintptr_t)addr % PAGE_SIZE);
+		if (size > MAXSIZE)
+			size = MAXSIZE;
+	}
+	totalsize = size;
 	buf = malloc(totalsize);
 	if (buf == NULL)
 		return (NULL);
 	for (;;) {
-		diff = totalsize - size;
 		iorequest.piod_op = PIOD_READ_D;
-		iorequest.piod_offs = (char *)offset + diff;
-		iorequest.piod_addr = buf + diff;
+		iorequest.piod_offs = (char *)addr + offset;
+		iorequest.piod_addr = buf + offset;
 		iorequest.piod_len = size;
 		if (ptrace(PT_IO, pid, (caddr_t)&iorequest, 0) < 0) {
 			free(buf);
 			return (NULL);
 		}
-		for (i = 0 ; i < size; i++) {
-			if (buf[diff + i] == '\0')
+		if (memchr(buf + offset, '\0', size) != NULL)
+			return (buf);
+		offset += size;
+		if (totalsize < MAXSIZE && max == 0) {
+			size = MAXSIZE - totalsize;
+			if (size > PAGE_SIZE)
+				size = PAGE_SIZE;
+			nbuf = realloc(buf, totalsize + size);
+			if (nbuf == NULL) {
+				buf[totalsize - 1] = '\0';
 				return (buf);
-		}
-		if (totalsize < MAXSIZE - BLOCKSIZE && max == 0) {
-			totalsize += BLOCKSIZE;
-			buf = realloc(buf, totalsize);
-			size = BLOCKSIZE;
+			}
+			buf = nbuf;
+			totalsize += size;
 		} else {
 			buf[totalsize - 1] = '\0';
 			return (buf);