linux-kernel - Re: [PATCH] nvdimm: Replace the usage of a variable by a direct function call in nd_pfn

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Date:   Fri, 14 Apr 2023 08:22:34 -0700
From:   Alison Schofield <alison.schofield@...el.com>
To:     Markus Elfring <Markus.Elfring@....de>
Cc:     kernel-janitors@...r.kernel.org, nvdimm@...ts.linux.dev,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Vishal Verma <vishal.l.verma@...el.com>, cocci@...ia.fr,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] nvdimm: Replace the usage of a variable by a direct
 function call in nd_pfn_validate()

On Fri, Apr 14, 2023 at 12:12:37PM +0200, Markus Elfring wrote:
> Date: Fri, 14 Apr 2023 12:01:15 +0200
> 
> The address of a data structure member was determined before
> a corresponding null pointer check in the implementation of
> the function “nd_pfn_validate”.
> 
> Thus avoid the risk for undefined behaviour by replacing the usage of
> the local variable “parent_uuid” by a direct function call within
> a later condition check.

Hi Markus,

I think I understand what you are saying above, but I don't follow
how that applies here. This change seems to be a nice simplification,
parent_uuid, is used once, just grab it when needed.

What is the risk of undefined behavior?

> 
> This issue was detected by using the Coccinelle software.
Which cocci script?

> 
> Fixes: d1c6e08e7503649e4a4f3f9e700e2c05300b6379 ("libnvdimm/labels: Add uuid helpers")

This fixes tag seems to be the wrong tag. It is a tag from when the
uuid helpers were introduce, not where parent_uuid was first introduced
and used. I'm not clear this warrants a Fixes tag anyway. Is there
really a bug here? Perhaps I'm missing something in the previous
explanation of risk.

checkpatch is WARNING on the tag format:
WARNING: Please use correct Fixes: style 'Fixes: <12 chars of sha1> ("<title line>")' - ie: 'Fixes: d1c6e08e7503 ("libnvdimm/labels: Add uuid helpers")'
#17:
    Fixes: d1c6e08e7503649e4a4f3f9e700e2c05300b6379 ("libnvdimm/labels: Add uuid helpers")

checkpatch is also WARNING on the commit msg:
WARNING: Possible unwrapped commit description (prefer a maximum 75 chars per line)
#5:
    nvdimm: Replace the usage of a variable by a direct function call in nd_pfn_validate()

Also, possible only my pet peeve, the long commit message spoils my
pretty 80 column view. Please trim it to not wrap here:

$git log --oneline pfn_devs.c
52b639e56a46 nvdimm: Replace the usage of a variable by a direct function call in nd_pfn_validate()
c91d71363084 nvdimm: Support sizeof(struct page) > MAX_STRUCT_PAGE_SIZE
6e9f05dc66f9 libnvdimm/pfn_dev: increase MAX_STRUCT_PAGE_SIZE
81beea55cb74 nvdimm: Drop nd_device_lock()
4a0079bc7aae nvdimm: Replace lockdep_mutex with local lock classes
322cbb50de71 block: remove genhd.h

Alison


> Signed-off-by: Markus Elfring <elfring@...rs.sourceforge.net>
> ---
>  drivers/nvdimm/pfn_devs.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/nvdimm/pfn_devs.c b/drivers/nvdimm/pfn_devs.c
> index af7d9301520c..f14cbfa500ed 100644
> --- a/drivers/nvdimm/pfn_devs.c
> +++ b/drivers/nvdimm/pfn_devs.c
> @@ -456,7 +456,6 @@ int nd_pfn_validate(struct nd_pfn *nd_pfn, const char *sig)
>  	unsigned long align, start_pad;
>  	struct nd_pfn_sb *pfn_sb = nd_pfn->pfn_sb;
>  	struct nd_namespace_common *ndns = nd_pfn->ndns;
> -	const uuid_t *parent_uuid = nd_dev_to_uuid(&ndns->dev);
> 
>  	if (!pfn_sb || !ndns)
>  		return -ENODEV;
> @@ -476,7 +475,7 @@ int nd_pfn_validate(struct nd_pfn *nd_pfn, const char *sig)
>  		return -ENODEV;
>  	pfn_sb->checksum = cpu_to_le64(checksum);
> 
> -	if (memcmp(pfn_sb->parent_uuid, parent_uuid, 16) != 0)
> +	if (memcmp(pfn_sb->parent_uuid, nd_dev_to_uuid(&ndns->dev), 16) != 0)
>  		return -ENODEV;
> 
>  	if (__le16_to_cpu(pfn_sb->version_minor) < 1) {
> --
> 2.40.0
>