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.

1 files changed, 29 insertions, 17 deletions

diff --git a/usr.bin/truss/syscalls.c b/usr.bin/truss/syscalls.c
index aa4bac5..b758ddb 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);