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

[Avi Weiss]

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 ;-)

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