Re: [PATCH] linux-user: Don't assume 0 is not a valid host timer_t value

[Daniel P . Berrangé]

On Mon, Jul 25, 2022 at 12:00:35PM +0100, Peter Maydell wrote:
> For handling guest POSIX timers, we currently use an array
> g_posix_timers[], whose entries are a host timer_t value, or 0 for
> "this slot is unused".  When the guest calls the timer_create syscall
> we look through the array for a slot containing 0, and use that for
> the new timer.
> 
> This scheme assumes that host timer_t values can never be zero.  This
> is unfortunately not a valid assumption -- for some host libc
> versions, timer_t values are simply indexes starting at 0.  When
> using this kind of host libc, the effect is that the first and second
> timers end up sharing a slot, and so when the guest tries to operate
> on the first timer it changes the second timer instead.

For sake of historical record, could you mention here which specific
libc impl / version highlights the problem.

> 
> Rework the timer allocation code, so that:
>  * the 'slot in use' indication uses a separate array from the
>    host timer_t array
>  * we grab the free slot atomically, to avoid races when multiple
>    threads call timer_create simultaneously
>  * releasing an allocated slot is abstracted out into a new
>    free_host_timer_slot() function called in the correct places
> 
> This fixes:
>  * problems on hosts where timer_t 0 is valid
>  * the FIXME in next_free_host_timer() about locking
>  * bugs in the error paths in timer_create where we forgot to release
>    the slot we grabbed, or forgot to free the host timer
> 
> Reported-by: Jon Alduan <jon.alduan@gmail.com>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>  linux-user/syscall.c | 24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)

With regards,
Daniel
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