Re: [PATCH v4 26/39] unwind_user/sframe: Enable debugging in uaccess regions

[Jens Remus]

On 22.01.2025 03:31, Josh Poimboeuf wrote:
> Objtool warns about calling pr_debug() from uaccess-enabled regions, and
> rightfully so.  Add a dbg_sec_uaccess() macro which temporarily disables
> uaccess before doing the dynamic printk, and use that to add debug
> messages throughout the uaccess-enabled regions.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

> diff --git a/kernel/unwind/sframe.c b/kernel/unwind/sframe.c

> @@ -53,12 +53,15 @@ static __always_inline int __read_fde(struct sframe_section *sec,
>   			      sizeof(struct sframe_fde), Efault);
>   
>   	ip = sec->sframe_start + fde->start_addr;
> -	if (ip < sec->text_start || ip > sec->text_end)
> +	if (ip < sec->text_start || ip > sec->text_end) {
> +		dbg_sec_uaccess("bad fde num %d\n", fde_num);
>   		return -EINVAL;
> +	}
>   
>   	return 0;
>   
>   Efault:
> +	dbg_sec_uaccess("fde %d usercopy failed\n", fde_num);
>   	return -EFAULT;
>   }

Add a similar debug message for SFRame FDE user copy failures?

diff --git a/kernel/unwind/sframe.c b/kernel/unwind/sframe.c

@@ -125,6 +125,7 @@ static __always_inline int __find_fde(struct sframe_section *sec,
        return 0;

 Efault:
+       dbg_sec_uaccess("fde usercopy failed\n");
        return -EFAULT;
 }


Printing the IP is probably not an option due to security concerns?
Printing the the CFA, FP, and RA offsets is too much traffic?  To debug
issues on s390 I had to add tons of additional debug messages to make
sense of what was actually going on.

Regards,
Jens
--
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@xxxxxxxxxx

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