[Libvir] [PATCH] Don't access line[-1] for a zero-length "line" from fgets.

[Jim Meyering]

In reviewing the StorageAPI patches, I saw that some bogus input could
cause a segfault.  There's similar code in libvirt.c, so this fixes both.
Note that virsh.c does this, too, but it already has the required guard.

The src/libvirt.c patch applies to the trunk, but isn't a big
deal, since it's in the sample AuthCallback function.

The storage_backend.c part applies to the work-in-progress
mercurial queue, but does matter, because we have no guarantee
what the exec'd command will output.


	Don't access line[-1] for a zero-length "line" from fgets.
	A NUL byte at beginning of input, or just after a newline
	would provoke an invalid buf[-1] access (possible segfault).
	* src/libvirt.c (virConnectAuthCallbackDefault):
	* src/storage_backend.c (virStorageBackendRunProgRegex):

Signed-off-by: Jim Meyering <meyering at redhat.com>
---
 src/libvirt.c         |    6 ++++--
 src/storage_backend.c |    2 +-
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/libvirt.c b/src/libvirt.c
index 2972382..331c937 100644
--- a/src/libvirt.c
+++ b/src/libvirt.c
@@ -74,6 +74,7 @@ static int virConnectAuthCallbackDefault(virConnectCredentialPtr cred,
     for (i = 0 ; i < ncred ; i++) {
         char buf[1024];
         char *bufptr = buf;
+        size_t len;

         if (printf("%s:", cred[i].prompt) < 0)
             return -1;
@@ -92,8 +93,9 @@ static int virConnectAuthCallbackDefault(virConnectCredentialPtr cred,
                 }
                 return -1;
             }
-            if (buf[strlen(buf)-1] == '\n')
-                buf[strlen(buf)-1] = '\0';
+            len = strlen(buf);
+            if (len != 0 && buf[len-1] == '\n')
+                buf[len-1] = '\0';
             break;

         case VIR_CRED_PASSPHRASE:
diff --git a/src/storage_backend.c b/src/storage_backend.c
index 32a8d65..58d8642 100644
--- a/src/storage_backend.c
+++ b/src/storage_backend.c
@@ -338,7 +338,7 @@ int virStorageBackendRunProgRegex(virConnectPtr conn,
     while (fgets(line, sizeof(line), list) != NULL) {
         /* Strip trailing newline */
         int len = strlen(line);
-        if (line[len-1] == '\n')
+        if (len && line[len-1] == '\n')
             line[len-1] = '\0';

         for (i = 0 ; i <= maxReg && i < nregex ; i++) {
--
1.5.4.rc4.1.g1895