[PATCH v2] iohelper: fix reading with O_DIRECT

[Nikolay Shirokovskiy <nshirokovskiy@xxxxxxxxxxxxx>]

saferead is not suitable for direct reads. If file size is not multiple
of align size then we get EINVAL on the read(2) that is supposed to
return 0 because read buffer will not be aligned at this point.

Let's not read again after partial read and check that we read
everything by comparing the number of total bytes read against file size.
---

Diff from v1:
- a couple of cosmetic changes

 src/util/iohelper.c | 42 +++++++++++++++++++++++++++++++++++++-----
 1 file changed, 37 insertions(+), 5 deletions(-)

diff --git a/src/util/iohelper.c b/src/util/iohelper.c
index fe15a92..9aa6ae0 100644
--- a/src/util/iohelper.c
+++ b/src/util/iohelper.c
@@ -56,6 +56,7 @@ runIO(const char *path, int fd, int oflags)
     unsigned long long total = 0;
     bool direct = O_DIRECT && ((oflags & O_DIRECT) != 0);
     off_t end = 0;
+    off_t cur;
 
 #if HAVE_POSIX_MEMALIGN
     if (posix_memalign(&base, alignMask + 1, buflen)) {
@@ -78,10 +79,22 @@ runIO(const char *path, int fd, int oflags)
         fdoutname = "stdout";
         /* To make the implementation simpler, we give up on any
          * attempt to use O_DIRECT in a non-trivial manner.  */
-        if (direct && ((end = lseek(fd, 0, SEEK_CUR)) != 0)) {
-            virReportSystemError(end < 0 ? errno : EINVAL, "%s",
-                                 _("O_DIRECT read needs entire seekable file"));
-            goto cleanup;
+        if (direct) {
+            if  ((cur = lseek(fd, 0, SEEK_CUR)) != 0) {
+                virReportSystemError(cur < 0 ? errno : EINVAL, "%s",
+                                     _("O_DIRECT read needs entire seekable file"));
+                goto cleanup;
+            }
+
+            if ((end = lseek(fd, 0, SEEK_END)) < 0) {
+                virReportSystemError(errno, "%s", _("can not seek file end"));
+                goto cleanup;
+            }
+
+            if (lseek(fd, cur, SEEK_SET) < 0) {
+                virReportSystemError(errno, "%s", _("can not seek file begin"));
+                goto cleanup;
+            }
         }
         break;
     case O_WRONLY:
@@ -109,7 +122,26 @@ runIO(const char *path, int fd, int oflags)
     while (1) {
         ssize_t got;
 
-        if ((got = saferead(fdin, buf, buflen)) < 0) {
+        /* in case of O_DIRECT we cannot read again to check for EOF
+         * after partial buffer read as it is done in saferead */
+        if (direct && fdin == fd && end - total < buflen) {
+            if (total == end)
+                break;
+
+            while ((got = read(fd, buf, buflen)) < 0 && errno == EINTR)
+                ;
+
+            if (got < end - total) {
+                virReportError(VIR_ERR_INTERNAL_ERROR,
+                               _("Unable to read last chunk from %s"),
+                               fdinname);
+                goto cleanup;
+            }
+        } else {
+            got = saferead(fdin, buf, buflen);
+        }
+
+        if (got < 0) {
             virReportSystemError(errno, _("Unable to read %s"), fdinname);
             goto cleanup;
         }
-- 
1.8.3.1

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