[Xen-devel] [PATCH] tools/libxc: Implement writev_exact() in the same style as write_exact()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

This implementation of writev_exact() will cope with an iovcnt greater than
IOV_MAX because glibc will actually let this work anyway, and it is very
useful not to have to work about this in the caller of writev_exact().  The
caller is still required to ensure that the sum of iov_len's doesn't overflow
a ssize_t.

Promote the MAX() and MIN() macro definitions from xg_save_restore.h to
xc_private.h

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>

---

From strace()ing while writing into a pipe, I have observed quite a few cases
where writev() will perform a partial overall write, but return with len
exactly on an iov[] element boundary.

However, I can't find any guarantee that a partial write won't end midway
through an iov[] element, which is why the adjustment is present.
---
 tools/libxc/xc_private.c      |   34 ++++++++++++++++++++++++++++++++++
 tools/libxc/xc_private.h      |   12 ++++++++++++
 tools/libxc/xg_save_restore.h |    6 ------
 3 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/tools/libxc/xc_private.c b/tools/libxc/xc_private.c
index a3da614..b11d5e5 100644
--- a/tools/libxc/xc_private.c
+++ b/tools/libxc/xc_private.c
@@ -854,6 +854,40 @@ int write_exact(int fd, const void *data, size_t size)
     return 0;
 }
 
+int writev_exact(int fd, struct iovec *iov, int iovcnt)
+{
+    int iov_idx = 0;
+    ssize_t len;
+
+    while ( iov_idx < iovcnt )
+    {
+        /* Skip over iov[] enties with 0 length. */
+        while ( iov[iov_idx].iov_len == 0 )
+            if ( ++iov_idx == iovcnt )
+                return 0;
+
+        len = writev(fd, &iov[iov_idx], MIN(iovcnt - iov_idx, IOV_MAX));
+
+        if ( (len == -1) && (errno == EINTR) )
+            continue;
+        if ( len <= 0 )
+            return -1;
+
+        /* Check iov[] to see whether we had a partial or complete write. */
+        while ( len > 0 && (iov_idx < iovcnt) )
+            len -= iov[iov_idx++].iov_len;
+
+        if ( len < 0 ) /* Partial write of iov[iov_idx - 1]. */
+        {
+            iov_idx--;
+            iov[iov_idx].iov_base += iov[iov_idx].iov_len + len;
+            iov[iov_idx].iov_len = -len;
+        }
+    }
+
+    return 0;
+}
+
 int xc_ffs8(uint8_t x)
 {
     int i;
diff --git a/tools/libxc/xc_private.h b/tools/libxc/xc_private.h
index c7730f2..431ab5e 100644
--- a/tools/libxc/xc_private.h
+++ b/tools/libxc/xc_private.h
@@ -28,6 +28,7 @@
 #include <sys/stat.h>
 #include <stdlib.h>
 #include <sys/ioctl.h>
+#include <sys/uio.h>
 
 #include "xenctrl.h"
 #include "xenctrlosdep.h"
@@ -343,6 +344,10 @@ int xc_flush_mmu_updates(xc_interface *xch, struct xc_mmu 
*mmu);
 /* Return 0 on success; -1 on error setting errno. */
 int read_exact(int fd, void *data, size_t size); /* EOF => -1, errno=0 */
 int write_exact(int fd, const void *data, size_t size);
+int writev_exact(int fd, struct iovec *iov, int iovcnt);
+/* Note - writev_exact() might modify iov.  Whether it does so in practice
+ * depends on whether your system implementation of writev() returns from a
+ * partial write in the middle of an iov element. */
 
 int xc_ffs8(uint8_t x);
 int xc_ffs16(uint16_t x);
@@ -352,4 +357,11 @@ int xc_ffs64(uint64_t x);
 #define DOMPRINTF(fmt, args...) xc_dom_printf(dom->xch, fmt, ## args)
 #define DOMPRINTF_CALLED(xch) xc_dom_printf((xch), "%s: called", __FUNCTION__)
 
+#ifndef MAX
+#define MAX(_a, _b) ((_a) >= (_b) ? (_a) : (_b))
+#endif
+#ifndef MIN
+#define MIN(_a, _b) ((_a) <= (_b) ? (_a) : (_b))
+#endif
+
 #endif /* __XC_PRIVATE_H__ */
diff --git a/tools/libxc/xg_save_restore.h b/tools/libxc/xg_save_restore.h
index aa93c13..34019fa 100644
--- a/tools/libxc/xg_save_restore.h
+++ b/tools/libxc/xg_save_restore.h
@@ -393,9 +393,3 @@ static inline int get_platform_info(xc_interface *xch, 
uint32_t dom,
         memset(&(_p)->x32._f[0], (_v), sizeof((_p)->x32._f));      \
 } while (0)
 
-#ifndef MAX
-#define MAX(_a, _b) ((_a) >= (_b) ? (_a) : (_b))
-#endif
-#ifndef MIN
-#define MIN(_a, _b) ((_a) <= (_b) ? (_a) : (_b))
-#endif
-- 
1.7.10.4


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