[PATCH 2/3] storage: avoid short reads while chasing backing chain

[Eric Blake <eblake@xxxxxxxxxx>]

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.

* 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@xxxxxxxxxx>
---

Found by inspection, rather than an actual EINTR, so I have no
idea how likely the bug was to hit in practice.  But better safe
than sorry!

 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 ad8a79d..398b0c0 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 4882006..0e8d52d 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 ff84719..10cf8bd 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 589778c..bbd5a7e 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;
     }
-- 
1.8.3.1

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