From 5327fad4f292e4f3f84884ffe158c492bf00519c Mon Sep 17 00:00:00 2001
From: Eric Blake <eblake@redhat.com>
Date: Tue, 5 Nov 2013 10:30:56 -0700
Subject: [PATCH] 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>
---
 src/libvirt_private.syms  |  1 +
 src/util/virfile.c        | 21 +++++++++++++++++++++
 src/util/virfile.h        |  9 ++++++---
 src/util/virstoragefile.c | 10 ++--------
 4 files changed, 30 insertions(+), 11 deletions(-)

diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
index ad8a79d7dc..398b0c08d1 100644
--- a/src/libvirt_private.syms
+++ b/src/libvirt_private.syms
@@ -1208,6 +1208,7 @@ virFileOpenAs;
 virFileOpenTty;
 virFilePrintf;
 virFileReadAll;
+virFileReadHeaderFD;
 virFileReadLimFD;
 virFileResolveAllLinks;
 virFileResolveLink;
diff --git a/src/util/virfile.c b/src/util/virfile.c
index 4882006d98..0e8d52d308 100644
--- 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 ff84719ccf..10cf8bd9dc 100644
--- 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 589778c090..bbd5a7e234 100644
--- a/src/util/virstoragefile.c
+++ b/src/util/virstoragefile.c
@@ -793,10 +793,7 @@ 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, &buf)) < 0) {
         virReportSystemError(errno, _("cannot read header '%s'"), path);
         goto cleanup;
     }
@@ -939,15 +936,12 @@ 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, &head)) < 0) {
         virReportSystemError(errno, _("cannot read header '%s'"), path);
         goto cleanup;
     }