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 d25397ce826dc1e754febf9303883df22594fd65..e5fca898fb773aab6a5f98f7c043c6ab82e8b4cb 100644 (file)
--- a/src/libvirt_private.syms
+++ b/src/libvirt_private.syms
@@ -1388,6 +1388,7 @@ virFileOpenAs;
 virFileOpenTty;
 virFilePrintf;
 virFileReadAll;
+virFileReadHeaderFD;
 virFileReadLimFD;
 virFileResolveAllLinks;
 virFileResolveLink;

diff --git a/src/util/virfile.c b/src/util/virfile.c
index 33a8a6d3f5713f1ddb95bf63c69b6b21ff2a5adb..6ed8db0c67b77602cd7910f1f1cc3e9a4ad495f4 100644 (file)
--- a/src/util/virfile.c
+++ b/src/util/virfile.c
@@ -1153,6 +1153,27 @@ 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 945359931defd92ac54385b5d42c8d255d1b555b..fc8aa90b25dcadd3b521412af7e4e926b468ada1 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;
     }