Re: [PATCH for 5.5 1/2 v2] rseq: Fix: Clarify rseq.h UAPI rseq_cs memory reclaim requirements

[Mathieu Desnoyers]

----- On Dec 20, 2019, at 3:33 PM, Mathieu Desnoyers mathieu.desnoyers@xxxxxxxxxxxx wrote:

> The rseq.h UAPI documents that the rseq_cs field must be cleared
> before reclaiming memory that contains the targeted struct rseq_cs.
> 
> We should extend this comment to also dictate that the rseq_cs field
> must be cleared before reclaiming memory of the code pointed to by
> the rseq_cs start_ip and post_commit_offset fields.
> 
> While we can expect that use of dlclose(3) will typically unmap
> both struct rseq_cs and its associated code at once, nothing would
> theoretically prevent a JIT from reclaiming the code without
> reclaiming the struct rseq_cs, which would erroneously allow the
> kernel to consider new code which is not a rseq critical section
> as a rseq critical section following a code reclaim.

Hi Peter,

Is there anything preventing this rseq UAPI documentation fix from being merged ?

Thanks,

Mathieu

> 
> Suggested-by: Florian Weimer <fw@xxxxxxxxxxxxx>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Florian Weimer <fw@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: "H . Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Paul Turner <pjt@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Neel Natu <neelnatu@xxxxxxxxxx>
> Cc: linux-api@xxxxxxxxxxxxxxx
> ---
> include/uapi/linux/rseq.h | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
> index 9a402fdb60e9..d94afdfc4b7c 100644
> --- a/include/uapi/linux/rseq.h
> +++ b/include/uapi/linux/rseq.h
> @@ -100,7 +100,9 @@ struct rseq {
> 	 * instruction sequence block, as well as when the kernel detects that
> 	 * it is preempting or delivering a signal outside of the range
> 	 * targeted by the rseq_cs. Also needs to be set to NULL by user-space
> -	 * before reclaiming memory that contains the targeted struct rseq_cs.
> +	 * before reclaiming memory that contains the targeted struct rseq_cs
> +	 * or reclaiming memory that contains the code referred to by the
> +	 * start_ip and post_commit_offset fields of struct rseq_cs.
> 	 *
> 	 * Read and set by the kernel. Set by user-space with single-copy
> 	 * atomicity semantics. This field should only be updated by the
> --
> 2.17.1

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com