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: OOM error reporting & buffer signedness

Conflicts:
	src/libvirt_private.syms,
	src/util/virfile.c,
	src/util/virfile.h: Moved code to virutil.{c,h}

src/libvirt_private.syms

@@ -1880,6 +1880,7 @@ virFileMatchesNameSuffix;
 virFileOpenAs;
 virFileOpenTty;
 virFileReadAll;
+virFileReadHeaderFD;
 virFileReadLimFD;
 virFileResolveAllLinks;
 virFileResolveLink;

src/util/virstoragefile.c

@@ -707,12 +707,7 @@ virStorageFileGetMetadataInternal(const char *path,
         goto cleanup;
     }
 
-    if (VIR_ALLOC_N(buf, len) < 0) {
-        virReportOOMError();
-        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;
     }
@@ -849,17 +844,12 @@ virStorageFileProbeFormatFromFD(const char *path, int fd)
         return VIR_STORAGE_FILE_DIR;
     }
 
-    if (VIR_ALLOC_N(head, len) < 0) {
-        virReportOOMError();
-        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;
     }

src/util/virutil.c

@@ -439,6 +439,25 @@ 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

src/util/virutil.h

@@ -58,6 +58,9 @@ int virSetUIDGIDWithCaps(uid_t uid, gid_t gid, gid_t *groups, int ngroups,
                          unsigned long long capBits,
                          bool clearExistingCaps);
 
+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;
 
 int virFileReadAll(const char *path, int maxlen, char **buf) ATTRIBUTE_RETURN_CHECK;