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

[Bradley Morgan]

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