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staging: comedi: fix extreme case of comedi_nsamples_left() 

`comedi_nsamples_left(s, nsamples)` returns the number of samples
remaining to complete an asynchronous command or the passed in
`nsamples`, whichever is lower.  However, it goes wrong in the extreme
case of setting the `nsamples` parameter to `UINT_MAX` when the number
of conversions per "scan" (`s->async->cmd.scan_end_arg`) is 1.  It uses
`comedi_nscans_remaining(s, nscans)` to determine the number of scans
remaining, or the parameter `nscans`, whichever is lower.  To determine
the parameter `nscans`, it divides `nsamples` by the number of
conversions per scan and adds 1.  The addition of 1 is to avoid setting
the parameter `nscans` to 0, as `comedi_nscans_remaining(s, nscans)`
treats that value specially.  However in the extreme case where
`nsamples` is `UINT_MAX` and the number of samples per scan is 1, the
addition of 1 to `nscans` overflows, producing the unwanted 0.

Fix it by refactoring new a function `__comedi_nscans_remaining(s,
nscans)` out of `comedi_nscans_remaining(s, nscans)`.  The new function
does everything except the special handling when `nscans` is 0.  Change
`comedi_nsamples_remaining()` to call the new function without adding 1
to `nscans` to avoid the overflow.

This overflow bug doesn't affect any of the current COMEDI drivers.  I
stumbled across it while changing to one of the drivers.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Ian Abbott 2015-10-23 10:56:09 +01:00 committed by Greg Kroah-Hartman
parent b5c4e4e9d1
commit 92d354cbe9
1 changed files with 21 additions and 17 deletions
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38
drivers/staging/comedi/drivers.c
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@ -425,6 +425,24 @@ unsigned int comedi_bytes_per_scan(struct comedi_subdevice *s)
}
EXPORT_SYMBOL_GPL(comedi_bytes_per_scan);
static unsigned int __comedi_nscans_left(struct comedi_subdevice *s,
unsigned int nscans)
{
struct comedi_async *async = s->async;
struct comedi_cmd *cmd = &async->cmd;
if (cmd->stop_src == TRIG_COUNT) {
unsigned int scans_left = 0;
if (async->scans_done < cmd->stop_arg)
scans_left = cmd->stop_arg - async->scans_done;
if (nscans > scans_left)
nscans = scans_left;
}
return nscans;
}
/**
* comedi_nscans_left() - Return the number of scans left in the command
* @s: COMEDI subdevice.
@ -442,25 +460,12 @@ EXPORT_SYMBOL_GPL(comedi_bytes_per_scan);
unsigned int comedi_nscans_left(struct comedi_subdevice *s,
unsigned int nscans)
{
struct comedi_async *async = s->async;
struct comedi_cmd *cmd = &async->cmd;
if (nscans == 0) {
unsigned int nbytes = comedi_buf_read_n_available(s);
nscans = nbytes / comedi_bytes_per_scan(s);
}
if (cmd->stop_src == TRIG_COUNT) {
unsigned int scans_left = 0;
if (async->scans_done < cmd->stop_arg)
scans_left = cmd->stop_arg - async->scans_done;
if (nscans > scans_left)
nscans = scans_left;
}
return nscans;
return __comedi_nscans_left(s, nscans);
}
EXPORT_SYMBOL_GPL(comedi_nscans_left);
@ -479,9 +484,8 @@ unsigned int comedi_nsamples_left(struct comedi_subdevice *s,
struct comedi_cmd *cmd = &async->cmd;
if (cmd->stop_src == TRIG_COUNT) {
/* +1 to force comedi_nscans_left() to return the scans left */
unsigned int nscans = (nsamples / cmd->scan_end_arg) + 1;
unsigned int scans_left = comedi_nscans_left(s, nscans);
unsigned int nscans = nsamples / cmd->scan_end_arg;
unsigned int scans_left = __comedi_nscans_left(s, nscans);
unsigned int scan_pos =
comedi_bytes_to_samples(s, async->scan_progress);
unsigned long long samples_left = 0;