----- On Jul 2, 2018, at 6:45 PM, Linus Torvalds torvalds@xxxxxxxxxxxxxxxxxxxx wrote:
> On Mon, Jul 2, 2018 at 3:31 PM Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>>
>> Change the rseq ABI so rseq_cs start_ip, post_commit_offset and abort_ip
>> fields are seen as 64-bit fields by both 32-bit and 64-bit kernels rather
>> that ignoring the 32 upper bits on 32-bit kernels. This ensures we have a
>> consistent behavior for a 32-bit binary executed on 32-bit kernels and in
>> compat mode on 64-bit kernels.
>
> Actually, now that I see this again, I react to:
>
>
>> +static int check_rseq_cs_padding(struct task_struct *t)
>> +{
>> + u32 pad;
>> + int ret;
>> +
>> + ret = __get_user(pad, &t->rseq->rseq_cs_padding);
>> + if (ret)
>> + return ret;
>> + if (pad)
>> + return -EINVAL;
>> + return 0;
>> +}
>
> This is all wrong.
>
> Just make "rseq_cs" be an __u64" too. That will clean up everything,
> and user space will have a much easier time filling it in too, since
> it's just one field. Instead of having to remember about the "let's
> fill in padding for 32-bit cases".
>
> Then the rseq_get_rseq_cs() will be
>
> __u64 rseq_cs;
>
> ret = get_user(rseq_cs, &t->rseq->rseq_cs);
> if (ret)
> return ret;
> ptr = (void *)rseq_cs;
> if (rseq_cs != (unsigned long)ptr)
> return -EINVAL;
>
> and it's all good, no #ifdef's etc needed.
>
> Hmm?
Unfortunately, that rseq->rseq_cs field needs to be updated by user-space
with single-copy atomicity. Therefore, we want 32-bit user-space to initialize
the padding with 0, and only update the low bits with single-copy atomicity.
>
> Sorry for the bike-shedding, but this is now the last remaining user
> of that LINUX_FIELD_u32_u64, so let's just get rid of it entirely, ok?
>
> Then we can also get rid of that silly uapi/linux/types_32_64.h header
> file entirely.
>
> That would be *lovely*. Simpler code, simpler and less error-prone
> interfaces, and one less specialized header file.
We can easily switch from LINUX_FIELD_u32_u64 to __u64 for fields within
struct rseq_cs because we have no requirement on update single-copy
atomicity. However, this is not true for the rseq->rseq_cs pointer.
Thoughts ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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