Re: [PATCH] blk-throttle: Calculate allowed value only when the throttle is enabled

[Yu Kuai]

Hi,

在 2023/10/06 0:24, Oleg Nesterov 写道:
> Hi Li,
>
> On 10/05, Li Nan wrote:
> > > I don't think this change is sufficient to prevent kernel crash, as a
> > > "clever" user could still set the bps_limit to U64_MAX - 1 (or another
> > > large value), which probably would still result in the same crash. The
> > > comment in mul_u64_u64_div_u64 suggests there's something we can do to
> > > better handle the overflow case, but I'm not sure what it's referring
> > > to. ("Will generate an #DE when the result doesn't fit u64, could fix
> > > with an __ex_table[] entry when it becomes an issue.") Otherwise, we
> >
> > When (a * mul) overflows, a divide 0 error occurs in
> > mul_u64_u64_div_u64(). Commit 3dc167ba5729 ("sched/cputime: Improve
> > cputime_adjust()") changed func and said: "Will generate an #DE when the
> > result doesn't fit u64, could fix with an __ex_table[] entry when it
> > becomes an issue." But we are unsure of how to fix it. Could you please
> > explain how to fix this issue.
>
> Not sure I understand the question...
>
> OK, we can change mul_u64_u64_div_u64() to trap the exception, say,
>
> 	static inline u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div)
> 	{
> 		u64 q;
>
> 		asm ("mulq %2; 1: divq %3; 2:\n"
> 		     _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_DEFAULT|EX_FLAG_CLEAR_AX)
> 					: "=a" (q)
> 					: "a" (a), "rm" (mul), "rm" (div)
> 					: "rdx");
>
> 		return q;
> 	}
>
> should (iiuc) return 0 if the result doesn't fit u64 or div == 0.
>
> But even if we forget that this is x86-specific, how can this help?
> What should calculate_bytes_allowed() do/return in this case?

I believe, U64_MAX should be returned if result doesn't fit u64;
> > > probably need to remove the mul_u64_u64_div_u64 and check for
> > > overflow/potential overflow ourselves?
>
> probably yes...

How about this?

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 1101fb6f6cc8..5482c316a103 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -723,6 +723,10 @@ static unsigned int calculate_io_allowed(u32 
iops_limit,

 static u64 calculate_bytes_allowed(u64 bps_limit, unsigned long 
jiffy_elapsed)
 {
+       if (jiffy_elapsed > HZ &&
+           bps_limit > mul_u64_u64_div_u64(U64_MAX, (u64)HZ, 
(u64)jiffy_elapsed);
+               return U64_MAX;
+
        return mul_u64_u64_div_u64(bps_limit, (u64)jiffy_elapsed, (u64)HZ);
 }

> Oleg.
>
> .