kldxref: Properly handle reading strings near the end of an ELF file

If a string is at or near the end of an input file and the amount of
remaining data in the file is smaller than the maximum string size,
the pread(2) system call would return a short read which is treated as
an error.  Instead, add a new helper function for reading a string
which permits short reads so long as the data read from the file
contains a terminated string.

Reported by:	jrtc27
Reviewed by:	jrtc27
Sponsored by:	University of Cambridge, Google, Inc.
Differential Revision:	https://reviews.freebsd.org/D44419

4 files changed, 24 insertions, 16 deletions

diff --git a/usr.sbin/kldxref/ef.c b/usr.sbin/kldxref/ef.c
index 1ef27f2bc54a..77ddada946f2 100644
--- a/usr.sbin/kldxref/ef.c
+++ b/usr.sbin/kldxref/ef.c
@@ -549,7 +549,6 @@ static int
 ef_seg_read_string(elf_file_t ef, GElf_Addr address, size_t len, char *dest)
 {
 	GElf_Off ofs;
-	int error;
 
 	ofs = ef_get_offset(ef, address);
 	if (ofs == 0) {
@@ -559,13 +558,7 @@ ef_seg_read_string(elf_file_t ef, GElf_Addr address, size_t len, char *dest)
 		return (EFAULT);
 	}
 
-	error = elf_read_raw_data(ef->ef_efile, ofs, dest, len);
-	if (error != 0)
-		return (error);
-	if (strnlen(dest, len) == len)
-		return (EFAULT);
-
-	return (0);
+	return (elf_read_raw_string(ef->ef_efile, ofs, dest, len));
 }
 
 int
diff --git a/usr.sbin/kldxref/ef.h b/usr.sbin/kldxref/ef.h
index 25dc5216b169..9d3dc1b1b435 100644
--- a/usr.sbin/kldxref/ef.h
+++ b/usr.sbin/kldxref/ef.h
@@ -189,6 +189,10 @@ int	elf_read_raw_data(struct elf_file *efile, off_t offset, void *dst,
 int	elf_read_raw_data_alloc(struct elf_file *efile, off_t offset,
     size_t len, void **out);
 
+/* Reads a single string at the given offset from an ELF file. */
+int	elf_read_raw_string(struct elf_file *efile, off_t offset, char *dst,
+    size_t len);
+
 /*
  * Read relocated data from an ELF file and return it in a
  * dynamically-allocated buffer.  Note that no translation
diff --git a/usr.sbin/kldxref/ef_obj.c b/usr.sbin/kldxref/ef_obj.c
index 1274a14c10af..ac83137cd394 100644
--- a/usr.sbin/kldxref/ef_obj.c
+++ b/usr.sbin/kldxref/ef_obj.c
@@ -248,7 +248,6 @@ static int
 ef_obj_seg_read_string(elf_file_t ef, GElf_Addr address, size_t len, char *dest)
 {
 	GElf_Off ofs;
-	int error;
 
 	ofs = ef_obj_get_offset(ef, address);
 	if (ofs == 0) {
@@ -258,13 +257,7 @@ ef_obj_seg_read_string(elf_file_t ef, GElf_Addr address, size_t len, char *dest)
 		return (EFAULT);
 	}
 
-	error = elf_read_raw_data(ef->ef_efile, ofs, dest, len);
-	if (error != 0)
-		return (error);
-	if (strnlen(dest, len) == len)
-		return (EFAULT);
-
-	return (0);
+	return (elf_read_raw_string(ef->ef_efile, ofs, dest, len));
 }
 
 int
diff --git a/usr.sbin/kldxref/elf.c b/usr.sbin/kldxref/elf.c
index e5fe90169d2c..a93cf996ccc7 100644
--- a/usr.sbin/kldxref/elf.c
+++ b/usr.sbin/kldxref/elf.c
@@ -198,6 +198,24 @@ elf_read_raw_data_alloc(struct elf_file *efile, off_t offset, size_t len,
 }
 
 int
+elf_read_raw_string(struct elf_file *efile, off_t offset, char *dst, size_t len)
+{
+	ssize_t nread;
+
+	nread = pread(efile->ef_fd, dst, len, offset);
+	if (nread == -1)
+		return (errno);
+	if (nread == 0)
+		return (EIO);
+
+	/* A short read is ok so long as the data contains a terminator. */
+	if (strnlen(dst, nread) == nread)
+		return (EFAULT);
+
+	return (0);
+}
+
+int
 elf_read_data(struct elf_file *efile, Elf_Type type, off_t offset, size_t len,
     void **out)
 {