linux-kernel - Re: [PATCH v2 1/2] mm: add GFP mask param to strndup

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Message-Id: <20200821205154.365e230dbf6f2b93a47443c4@linux-foundation.org>
Date:   Fri, 21 Aug 2020 20:51:54 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Pascal Bouchareine <kalou@....net>
Cc:     linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        "Jakub Kicinski" <kuba@...nel.org>,
        "Alexey Dobriyan" <adobriyan@...il.com>,
        "Al Viro" <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v2 1/2] mm: add GFP mask param to strndup_user

On Fri, 21 Aug 2020 20:28:26 -0700 Pascal Bouchareine <kalou@....net> wrote:

> Let caller specify allocation.
> Preserve existing calls with GFP_USER.
> 
>  21 files changed, 65 insertions(+), 43 deletions(-)

Why change all existing callsites so that one callsite can pass in a
different gfp_t?

> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 1ca609f66fdf..3d94ba811f4b 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -326,7 +326,7 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
>   */
>  static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
>  {
> -	char *name = strndup_user(buf, DMA_BUF_NAME_LEN);
> +	char *name = strndup_user(buf, DMA_BUF_NAME_LEN, GFP_USER);
>  	long ret = 0;

Wouldn't

#include <linux/gfp.h>

char *__strndup_user(const char __user *s, long n, gfp_t gfp);

static inline char *strndup_user(const char __user *s, long n)
{
	return __strndup_user(s, n, GFP_USER);
}

be simpler?



Also...

why does strndup_user() use GFP_USER?  Nobody will be mapping the
resulting strings into user pagetables (will they?).  This was done by
Al's 6c2c97a24f096e32, which doesn't have a changelog :(


In [patch 2/2],

+	desc = strndup_user(user_desc, SK_MAX_DESC_SIZE, GFP_KERNEL_ACCOUNT);

if GFP_USER is legit then shouldn't this be GFP_USER_ACCOUNT (ie,
GFP_USER|__GFP_ACCOUNT)?