Tapdisk O_DIRECT breaks read-only-ness

Tapdisk O_DIRECT breaks read-only-ness

[Christopher Head]

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Hi all,
It looks like some time ago a patch was applied to the Tapdisk backend
drivers to add O_DIRECT to the open system calls whenever possible.
However, on line 141 of tools/blktap/drivers/block-sync.c, at least as
of revision 16947:f66bb3b80661 of Xen 3.2, it looks like there's a
slight logic error: this line is where the open() system call is
repeated without O_DIRECT (in case O_DIRECT fails), but instead of
passing in o_flags as the second parameter, O_RDWR|O_LARGEFILE is passed
directly. This means that if tap:sync is unable to open a file with
O_DIRECT, it will then attempt to open the file read-write even if the
original request was for a read-only open.

Chris
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Re: Tapdisk O_DIRECT breaks read-only-ness

[Samuel Thibault]

Christopher Head, le Thu 10 Jul 2008 21:02:06 -0700, a écrit :
> However, on line 141 of tools/blktap/drivers/block-sync.c, at least as
> of revision 16947:f66bb3b80661 of Xen 3.2, it looks like there's a
> slight logic error: this line is where the open() system call is
> repeated without O_DIRECT (in case O_DIRECT fails), but instead of
> passing in o_flags as the second parameter, O_RDWR|O_LARGEFILE is passed
> directly.



tapdisk: in case O_DIRECT fails, retry again with o_flags but O_DIRECT,
instead of fixed value O_RD_WR | O_LARGEFILE

Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>

diff -r 06d80308aa2e tools/blktap/drivers/block-sync.c
--- a/tools/blktap/drivers/block-sync.c	Thu Jul 10 19:23:55 2008 +0100
+++ b/tools/blktap/drivers/block-sync.c	Fri Jul 11 11:21:58 2008 +0100
@@ -129,7 +129,7 @@
 
                 /* Maybe O_DIRECT isn't supported. */
 		o_flags &= ~O_DIRECT;
-                fd = open(name, O_RDWR | O_LARGEFILE);
+                fd = open(name, o_flags);
                 if (fd != -1) DPRINTF("WARNING: Accessing image without"
                                      "O_DIRECT! (%s)\n", name);