Re: [PATCH v12 05/28] riscv: usercfi state for task and save/restore of CSR_SSP on trap entry/exit

[Radim Krčmář]

2025-04-23T17:00:29-07:00, Deepak Gupta <debug@xxxxxxxxxxxx>:
> On Thu, Apr 10, 2025 at 01:04:39PM +0200, Radim Krčmář wrote:
>>2025-03-14T14:39:24-07:00, Deepak Gupta <debug@xxxxxxxxxxxx>:
>>> diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
>>> @@ -62,6 +62,9 @@ struct thread_info {
>>>  	long			user_sp;	/* User stack pointer */
>>>  	int			cpu;
>>>  	unsigned long		syscall_work;	/* SYSCALL_WORK_ flags */
>>> +#ifdef CONFIG_RISCV_USER_CFI
>>> +	struct cfi_status	user_cfi_state;
>>> +#endif
>>
>>I don't think it makes sense to put all the data in thread_info.
>>kernel_ssp and user_ssp is more than enough and the rest can comfortably
>>live elsewhere in task_struct.
>>
>>thread_info is supposed to be as small as possible -- just spanning
>>multiple cache-lines could be noticeable.
>
> I can change it to only include only `user_ssp`, base and size.

No need for base and size either -- we don't touch that in the common
exception code.

> But before we go there, see below:
>
> $ pahole -C thread_info kbuild/vmlinux
> struct thread_info {
>          long unsigned int          flags;                /*     0     8 */
>          int                        preempt_count;        /*     8     4 */
>
>          /* XXX 4 bytes hole, try to pack */
>
>          long int                   kernel_sp;            /*    16     8 */
>          long int                   user_sp;              /*    24     8 */
>          int                        cpu;                  /*    32     4 */
>
>          /* XXX 4 bytes hole, try to pack */
>
>          long unsigned int          syscall_work;         /*    40     8 */
>          struct cfi_status          user_cfi_state;       /*    48    32 */
>          /* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
>          long unsigned int          a0;                   /*    80     8 */
>          long unsigned int          a1;                   /*    88     8 */
>          long unsigned int          a2;                   /*    96     8 */
>
>          /* size: 104, cachelines: 2, members: 10 */
>          /* sum members: 96, holes: 2, sum holes: 8 */
>          /* last cacheline: 40 bytes */
> };
>
> If we were to remove entire `cfi_status`, it would still be 72 bytes (88 bytes
> if shadow call stack were enabled) and already spans across two cachelines.

It has only 64 bytes of data without shadow call stack, but it wasted 8
bytes on the holes.
a2 is somewhat an outlier that is not used most exception paths and
excluding it makes everything fit nicely even now.

> if shadow call stack were enabled) and already spans across two cachelines. I
> did see the comment above that it should fit inside a cacheline. Although I
> assumed its stale comment given that it already spans across cacheline and I
> didn't see any special mention in commit messages of changes which grew this
> structure above one cacheline. So I assumed this was a stale comment.
>
> On the other hand, whenever enable/lock bits are checked, there is a high
> likelyhood that user_ssp and other fields are going to be accessed and
> thus it actually might be helpful to have it all in one cacheline during
> runtime.

Yes, although accessing enable/lock bits will be relatively rare.
It seems better to have the overhead during thread setup, rather than on
every trap.

> So I am not sure if its helpful sticking to the comment which already is stale.

We could fix the holes and also use sp instead of a0 in the
new_vmalloc_check, so everything would fit better.

We are really close to fitting into a single cache-line, so I'd prefer
if shadow stack only filled thread_info with data that is used very
often in the exception handling code.

I think we could do without user_sp in thread_info as well, so there are
other packing options.

Btw. could ssp be added to pt_regs?

Thanks.