Re: [PATCH] profiling: fix sampling step on 64-bit big-endian systems

[Bradley Morgan]

On 20 October 2026 17:47:22 BST, Avi Weiss <thnkslprpt@xxxxxxxxx> wrote:
>I'm happy to try it anyway - just so I know how for future patches.
>I'll see how many I can run through before my laptop gives up ;-)

sometimes the crux is for these kinds of patches something in some cryptic
arch gets messed up in vmlinux stage, odd right?

This *makes sense* for me, off the top of my head and thinking hard id say
this *shouldn't* do anything bad to a cryptic architecture, but I've been
shocked in the past.

>
>On Tue, 20 Oct 2026 at 18:07, Bradley Morgan <brads@xxxxxxxxxxxxxx> wrote:
>>
>> On 20 October 2026 11:12:24 BST, Avi Weiss <thnkslprpt@xxxxxxxxx> wrote:
>> >read_profile() stores the sampling step in an unsigned long
>> >but copies only four bytes to userspace. On 64-bit
>> >big-endian systems (not common obviously), this copies
>> >the upper 32 bits and discards teh rest, causing
>> >/proc/profile to report zero for normal shift values.
>> >
>> >Cast to unsigned int so we get the correct 4 bytes, while
>> >keeping the shift calculation in unsigned long.
>> >
>> >A small C reproducer under qemu-s390x reproduced this and
>> >proved the fix works for prof_shift=2.
>>
>> If you wanna be 100% sure that this is regression free, you could test
>> building on all arches. Sometimes linking fails in cases.
>>
>> >
>> >Fixes: 2d186afd04d6 ("profiling: fix shift-out-of-bounds bugs")
>> >Signed-off-by: Avi Weiss <thnkslprpt@xxxxxxxxx>
>> >---
>> >
>> > kernel/profile.c | 2 +-
>> > 1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> >diff --git a/kernel/profile.c b/kernel/profile.c
>> >index 1fcf1adcf4eb0..e7defa90a34c9 100644
>> >--- a/kernel/profile.c
>> >+++ b/kernel/profile.c
>> >@@ -156,7 +156,7 @@ read_profile(struct file *file, char __user *buf,
>size_t count, loff_t *ppos)
>> >       unsigned long p = *ppos;
>> >       ssize_t read;
>> >       char *pnt;
>> >-      unsigned long sample_step = 1UL << prof_shift;
>> >+      unsigned int sample_step = (unsigned int)(1UL << prof_shift);
>>
>> But this would compile anyway.
>>
>> Reviewed-by: Bradley Morgan <brads@xxxxxxxxxxxxxx>
>>
>>
>> >
>> >       if (p >= (prof_len+1)*sizeof(unsigned int))
>> >               return 0;
>> >
>>
>> --- Thanks!
>> "I'm not a very positive person" - Linus torvalds
>

--- Thanks!
"I'm not a very positive person" - Linus torvalds