[PATCH] tools/xenstored: Don't assume errno will not be overwritten in lu_arch()

[Julien Grall <julien@xxxxxxx>]

From: Julien Grall <jgrall@xxxxxxxxxx>

At the moment, do_control_lu() will set errno to 0 before calling
lu_arch() and then check errno. The expectation is nothing in lu_arch()
will change the value unless there is an error.

However, per errno(3), a function that succeeds is allowed to change
errno. In fact, syslog() will overwrite errno if the logs are rotated
at the time it is called.

To prevent any further issue, errno is now always set before
returning NULL.

Additionally, errno is only checked when returning NULL so the client
can see the error message if there is any.

Reported-by: Michael Kurth <mku@xxxxxxxxxx>
Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
---
 tools/xenstore/xenstored_control.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/tools/xenstore/xenstored_control.c 
b/tools/xenstore/xenstored_control.c
index 6b68b79faac7..6fcb42095b59 100644
--- a/tools/xenstore/xenstored_control.c
+++ b/tools/xenstore/xenstored_control.c
@@ -324,6 +324,7 @@ static const char *lu_binary_alloc(const void *ctx, struct 
connection *conn,
        lu_status->kernel_size = size;
        lu_status->kernel_off = 0;
 
+       errno = 0;
        return NULL;
 }
 
@@ -339,6 +340,7 @@ static const char *lu_binary_save(const void *ctx, struct 
connection *conn,
        memcpy(lu_status->kernel + lu_status->kernel_off, data, size);
        lu_status->kernel_off += size;
 
+       errno = 0;
        return NULL;
 }
 
@@ -798,9 +800,8 @@ static int do_control_lu(void *ctx, struct connection *conn,
                if (!ret)
                        return errno;
        } else {
-               errno = 0;
                ret = lu_arch(ctx, conn, vec, num);
-               if (errno)
+               if (!ret && errno)
                        return errno;
        }
 
-- 
2.17.1