Tapdisk O_DIRECT breaks read-only-ness

[Christopher Head <chead@cs.ubc.ca>]

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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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