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.
.
