linux-kernel - Re: [PATCH] gcov: Add support for checksum field

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Message-ID: <1da3f764-4445-2135-3292-2912c2e8e778@linux.ibm.com>
Date:   Mon, 19 Dec 2022 17:48:14 +0100
From:   Peter Oberparleiter <oberpar@...ux.ibm.com>
To:     Rickard Andersson <rickaran@...s.com>, linux-kernel@...r.kernel.org
Cc:     rickard314.andersson@...il.com, mliska@...e.cz
Subject: Re: [PATCH] gcov: Add support for checksum field

On 19.12.2022 16:06, Rickard Andersson wrote:
> From: Rickard x Andersson <rickaran@...s.com>
> 
> In GCC version 12.1 a checksum field was added.

Thanks for the patch!

In another e-mail you mentioned that this patch fixes a kernel crash
during boot when using gcov-kernel with GCC 12. Please add this
information to the commit message and if possible the platform on which
this occurs.

Also this patch fixes a missing piece from a previous patch, so please add:

Fixes: 977ef30a7d88 ("gcov: support GCC 12.1 and newer compilers")
Cc: <stable@...r.kernel.org>

Finally I reviewed and tested the patch and it looks good to me, so
please add:

Reviewed-by: Peter Oberparleiter <oberpar@...ux.ibm.com>
Tested-by: Peter Oberparleiter <oberpar@...ux.ibm.com>

Please resend with these commit message changes. Thanks!

For the record: I wondered why my testing of the previous patch with GCC
12 didn't catch this. It turns out that this crash does not occur on
architectures with 8-byte pointer alignment such as s390x where I
performed my tests. Consider this pahole output on s390x without the patch:

struct gcov_info {
[...]
        unsigned int       stamp;                /*    16     4 */
        /* XXX 4 bytes hole, try to pack */
        const char  *      filename;             /*    24     8 */
[...]
}

And with the patch:

struct gcov_info {
[...]
       unsigned int       stamp;                /*    16     4 */
       unsigned int       checksum;             /*    20     4 */
       const char  *      filename;             /*    24     8 */
[...]
}

As can be seen, the offset of the filename and subsequent fields does
not change because the new field fills an alignment hole. It would
change (resulting in a crash during boot) if the alignment-requirement
of the const char *filename field would be different.

> 
> Signed-off-by: Rickard x Andersson <rickaran@...s.com>
> ---
>  kernel/gcov/gcc_4_7.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/gcov/gcc_4_7.c b/kernel/gcov/gcc_4_7.c
> index c699feda21ac..04880d8fba25 100644
> --- a/kernel/gcov/gcc_4_7.c
> +++ b/kernel/gcov/gcc_4_7.c
> @@ -85,6 +85,7 @@ struct gcov_fn_info {
>   * @version: gcov version magic indicating the gcc version used for compilation
>   * @next: list head for a singly-linked list
>   * @stamp: uniquifying time stamp
> + * @checksum: unique object checksum
>   * @filename: name of the associated gcov data file
>   * @merge: merge functions (null for unused counter type)
>   * @n_functions: number of instrumented functions
> @@ -97,6 +98,10 @@ struct gcov_info {
>  	unsigned int version;
>  	struct gcov_info *next;
>  	unsigned int stamp;
> + /* Since GCC 12.1 a checksum field is added. */
> +#if (__GNUC__ >= 12)
> +	unsigned int checksum;
> +#endif
>  	const char *filename;
>  	void (*merge[GCOV_COUNTERS])(gcov_type *, unsigned int);
>  	unsigned int n_functions;

-- 
Peter Oberparleiter
Linux on IBM Z Development - IBM Germany R&D