storage: avoid short reads while chasing backing chain

Our backing file chain code was not very robust to an ill-timed
EINTR, which could lead to a short read causing us to randomly
treat metadata differently than usual.  But the existing
virFileReadLimFD forces an error if we don't read the entire
file, even though we only care about the header of the file.
So add a new virFile function that does what we want.

* src/util/virfile.h (virFileReadHeaderFD): New prototype.
* src/util/virfile.c (virFileReadHeaderFD): New function.
* src/libvirt_private.syms (virfile.h): Export it.
* src/util/virstoragefile.c (virStorageFileGetMetadataInternal)
(virStorageFileProbeFormatFromFD): Use it.

Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit 5327fad4f292e4f3f84884ffe158c492bf00519c)

Conflicts:
	src/util/virstoragefile.c: buffer signedness

diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
index b7ed3bdcc38dc6ce230255263eb17ca0c8a0645e..fb889ed4d6304005735cbe0cd52a943fe2a19cc5 100644 (file)
--- a/src/libvirt_private.syms
+++ b/src/libvirt_private.syms
@@ -1202,6 +1202,7 @@ virFileOpenAs;
 virFileOpenTty;
 virFilePrintf;
 virFileReadAll;
+virFileReadHeaderFD;
 virFileReadLimFD;
 virFileResolveAllLinks;
 virFileResolveLink;

diff --git a/src/util/virfile.c b/src/util/virfile.c
index 3a9980cf4e4e4c7c2fc68402404f5e01983e34c5..63df029203a034d34106466811eb3f8547549d94 100644 (file)
--- a/src/util/virfile.c
+++ b/src/util/virfile.c
@@ -1225,6 +1225,27 @@ saferead_lim(int fd, size_t max_len, size_t *length)
     return NULL;
 }
 
+
+/* A wrapper around saferead_lim that merely stops reading at the
+ * specified maximum size.  */
+int
+virFileReadHeaderFD(int fd, int maxlen, char **buf)
+{
+    size_t len;
+    char *s;
+
+    if (maxlen <= 0) {
+        errno = EINVAL;
+        return -1;
+    }
+    s = saferead_lim(fd, maxlen, &len);
+    if (s == NULL)
+        return -1;
+    *buf = s;
+    return len;
+}
+
+
 /* A wrapper around saferead_lim that maps a failure due to
    exceeding the maximum size limitation to EOVERFLOW.  */
 int

diff --git a/src/util/virfile.h b/src/util/virfile.h
index 72d35ce4ebe9b76d9ddc489838c9f98b291119bb..e2e708acca965fc36a5f39aa27f112141471f5ab 100644 (file)
--- a/src/util/virfile.h
+++ b/src/util/virfile.h
@@ -122,9 +122,12 @@ int virFileNBDDeviceAssociate(const char *file,
 
 int virFileDeleteTree(const char *dir);
 
-int virFileReadLimFD(int fd, int maxlen, char **buf) ATTRIBUTE_RETURN_CHECK;
-
-int virFileReadAll(const char *path, int maxlen, char **buf) ATTRIBUTE_RETURN_CHECK;
+int virFileReadHeaderFD(int fd, int maxlen, char **buf)
+    ATTRIBUTE_RETURN_CHECK ATTRIBUTE_NONNULL(3);
+int virFileReadLimFD(int fd, int maxlen, char **buf)
+    ATTRIBUTE_RETURN_CHECK ATTRIBUTE_NONNULL(3);
+int virFileReadAll(const char *path, int maxlen, char **buf)
+    ATTRIBUTE_RETURN_CHECK ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(3);
 
 int virFileWriteStr(const char *path, const char *str, mode_t mode)
     ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2) ATTRIBUTE_RETURN_CHECK;

diff --git a/src/util/virstoragefile.c b/src/util/virstoragefile.c
index 1b4d4cf85e2ce69f27c3798765db4136c2d16c8b..9c4715aa06af5618e24f9147f1a7d5e4bded7add 100644 (file)
--- a/src/util/virstoragefile.c
+++ b/src/util/virstoragefile.c
@@ -788,10 +788,7 @@ virStorageFileGetMetadataInternal(const char *path,
         goto cleanup;
     }
 
-    if (VIR_ALLOC_N(buf, len) < 0)
-        goto cleanup;
-
-    if ((len = read(fd, buf, len)) < 0) {
+    if ((len = virFileReadHeaderFD(fd, len, (char **)&buf)) < 0) {
         virReportSystemError(errno, _("cannot read header '%s'"), path);
         goto cleanup;
     }
@@ -934,15 +931,12 @@ virStorageFileProbeFormatFromFD(const char *path, int fd)
         return VIR_STORAGE_FILE_DIR;
     }
 
-    if (VIR_ALLOC_N(head, len) < 0)
-        return -1;
-
     if (lseek(fd, 0, SEEK_SET) == (off_t)-1) {
         virReportSystemError(errno, _("cannot set to start of '%s'"), path);
         goto cleanup;
     }
 
-    if ((len = read(fd, head, len)) < 0) {
+    if ((len = virFileReadHeaderFD(fd, len, (char **)&head)) < 0) {
         virReportSystemError(errno, _("cannot read header '%s'"), path);
         goto cleanup;
     }