On 16.11.2013 22:37, Jonas Petersen wrote:
> By design the Xlib 32-bit internal request sequence numbers may wrap. There
> is two locations within xcb_io.c that are not wrap-safe. The value of
> last_flushed relies on the request to be sequential all the time. This is
> not given when the sequence has just wrapped. Applications may then crash
> with a "Fatal IO error 11 (Resource temporarily unavailable)".
>
> This patch fixes this by unwrapping the sequence number when needed to retain
> the sequence relative to last_flushed.
>
> Signed-off-by: Jonas Petersen <[email protected]>
> ---
> src/xcb_io.c | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/src/xcb_io.c b/src/xcb_io.c
> index 727c6c7..f2978d0 100644
> --- a/src/xcb_io.c
> +++ b/src/xcb_io.c
> @@ -455,7 +455,7 @@ void _XSend(Display *dpy, const char *data, long size)
> static const xReq dummy_request;
> static char const pad[3];
> struct iovec vec[3];
> - uint64_t requests;
> + uint64_t requests, unwrapped_request;
> _XExtension *ext;
> xcb_connection_t *c = dpy->xcb->connection;
> if(dpy->flags & XlibDisplayIOError)
> @@ -464,6 +464,10 @@ void _XSend(Display *dpy, const char *data, long size)
> if(dpy->bufptr == dpy->buffer && !size)
> return;
>
> + /* Set bit 8 of 'request' when a 32-bit wrap has just happened
> + * so the sequence stays correct relatively to 'last_flushed'. */
"1 << 32" does not set bit 8 and this doesn't set anything in request, really.
> + unwrapped_request = ((uint64_t)(dpy->request < dpy->xcb->last_flushed)
> << 32) + dpy->request;
Also, I don't think that this code is intuitive this way.
I would propose this instead:
unwrapped_request = dpy->request;
/* If there was a sequence number wrap since our last flush, make sure we
* use the correct 64 bit sequence number */
if (sizeof(uint64_t) > sizeof(unsigned long)
&& dpy->request < dpy->xcb_last_flushed)
unwrapped_request += UINT64_C(1) << 32;
(I am not sure/convinced if the sizeof comparision is necessary, but I saw
something like this in require_socket() and then thought that this might be
necessary on systems where unsigned long already is a 64 bit type.)
> /* iff we asked XCB to set aside errors, we must pick those up
> * eventually. iff there are async handlers, we may have just
> * issued requests that will generate replies. in either case,
> @@ -471,10 +475,10 @@ void _XSend(Display *dpy, const char *data, long size)
> if(dpy->xcb->event_owner != XlibOwnsEventQueue || dpy->async_handlers)
> {
> uint64_t sequence;
> - for(sequence = dpy->xcb->last_flushed + 1; sequence <=
> dpy->request; ++sequence)
> + for(sequence = dpy->xcb->last_flushed + 1; sequence <=
> unwrapped_request; ++sequence)
> append_pending_request(dpy, sequence);
> }
> - requests = dpy->request - dpy->xcb->last_flushed;
> + requests = unwrapped_request - dpy->xcb->last_flushed;
> dpy->xcb->last_flushed = dpy->request;
>
> vec[0].iov_base = dpy->buffer;
>
--
"Every once in a while, declare peace. It confuses the hell out of your enemies"
- 79th Rule of Acquisition
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