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Message-ID: <qdzuvazxkvueook2a64qo2jcdrrbnkp2pn2dbury34ad47jvno@eu6ul4fso6yi>
Date: Sat, 17 Aug 2024 11:05:06 +0200
From: Alejandro Colomar <alx@...nel.org>
To: Yafang Shao <laoar.shao@...il.com>
Cc: akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
justinstitt@...gle.com, ebiederm@...ssion.com, alexei.starovoitov@...il.com,
rostedt@...dmis.org, catalin.marinas@....com, penguin-kernel@...ove.sakura.ne.jp,
linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
linux-trace-kernel@...r.kernel.org, audit@...r.kernel.org, linux-security-module@...r.kernel.org,
selinux@...r.kernel.org, bpf@...r.kernel.org, netdev@...r.kernel.org,
dri-devel@...ts.freedesktop.org, Simon Horman <horms@...nel.org>,
Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v7 6/8] mm/util: Deduplicate code in
{kstrdup,kstrndup,kmemdup_nul}
On Sat, Aug 17, 2024 at 10:58:02AM GMT, Alejandro Colomar wrote:
> Hi Yafang,
>
> On Sat, Aug 17, 2024 at 10:56:22AM GMT, Yafang Shao wrote:
> > These three functions follow the same pattern. To deduplicate the code,
> > let's introduce a common helper __kmemdup_nul().
> >
> > Suggested-by: Andrew Morton <akpm@...ux-foundation.org>
> > Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> > Cc: Simon Horman <horms@...nel.org>
> > Cc: Matthew Wilcox <willy@...radead.org>
> > ---
> > mm/util.c | 67 +++++++++++++++++++++----------------------------------
> > 1 file changed, 26 insertions(+), 41 deletions(-)
> >
> > diff --git a/mm/util.c b/mm/util.c
> > index 4542d8a800d9..310c7735c617 100644
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -45,33 +45,40 @@ void kfree_const(const void *x)
> > EXPORT_SYMBOL(kfree_const);
> >
> > /**
> > - * kstrdup - allocate space for and copy an existing string
> > - * @s: the string to duplicate
> > + * __kmemdup_nul - Create a NUL-terminated string from @s, which might be unterminated.
> > + * @s: The data to copy
> > + * @len: The size of the data, including the null terminator
> > * @gfp: the GFP mask used in the kmalloc() call when allocating memory
> > *
> > - * Return: newly allocated copy of @s or %NULL in case of error
> > + * Return: newly allocated copy of @s with NUL-termination or %NULL in
> > + * case of error
> > */
> > -noinline
> > -char *kstrdup(const char *s, gfp_t gfp)
> > +static __always_inline char *__kmemdup_nul(const char *s, size_t len, gfp_t gfp)
> > {
> > - size_t len;
> > char *buf;
> >
> > - if (!s)
> > + buf = kmalloc_track_caller(len, gfp);
> > + if (!buf)
> > return NULL;
> >
> > - len = strlen(s) + 1;
> > - buf = kmalloc_track_caller(len, gfp);
> > - if (buf) {
> > - memcpy(buf, s, len);
> > - /* During memcpy(), the string might be updated to a new value,
> > - * which could be longer than the string when strlen() is
> > - * called. Therefore, we need to add a null termimator.
> > - */
> > - buf[len - 1] = '\0';
> > - }
> > + memcpy(buf, s, len);
> > + /* Ensure the buf is always NUL-terminated, regardless of @s. */
> > + buf[len - 1] = '\0';
> > return buf;
> > }
> > +
> > +/**
> > + * kstrdup - allocate space for and copy an existing string
> > + * @s: the string to duplicate
> > + * @gfp: the GFP mask used in the kmalloc() call when allocating memory
> > + *
> > + * Return: newly allocated copy of @s or %NULL in case of error
> > + */
> > +noinline
> > +char *kstrdup(const char *s, gfp_t gfp)
> > +{
> > + return s ? __kmemdup_nul(s, strlen(s) + 1, gfp) : NULL;
> > +}
> > EXPORT_SYMBOL(kstrdup);
> >
> > /**
> > @@ -106,19 +113,7 @@ EXPORT_SYMBOL(kstrdup_const);
> > */
> > char *kstrndup(const char *s, size_t max, gfp_t gfp)
> > {
> > - size_t len;
> > - char *buf;
> > -
> > - if (!s)
> > - return NULL;
> > -
> > - len = strnlen(s, max);
> > - buf = kmalloc_track_caller(len+1, gfp);
> > - if (buf) {
> > - memcpy(buf, s, len);
> > - buf[len] = '\0';
> > - }
> > - return buf;
> > + return s ? __kmemdup_nul(s, strnlen(s, max) + 1, gfp) : NULL;
> > }
> > EXPORT_SYMBOL(kstrndup);
> >
> > @@ -192,17 +187,7 @@ EXPORT_SYMBOL(kvmemdup);
> > */
> > char *kmemdup_nul(const char *s, size_t len, gfp_t gfp)
> > {
> > - char *buf;
> > -
> > - if (!s)
> > - return NULL;
> > -
> > - buf = kmalloc_track_caller(len + 1, gfp);
> > - if (buf) {
> > - memcpy(buf, s, len);
> > - buf[len] = '\0';
> > - }
> > - return buf;
> > + return s ? __kmemdup_nul(s, len + 1, gfp) : NULL;
> > }
> > EXPORT_SYMBOL(kmemdup_nul);
>
> I like the idea of the patch, but it's plagued with all those +1 and -1.
> I think that's due to a bad choice of value being passed by. If you
> pass the actual length of the string (as suggested in my reply to the
> previous patch) you should end up with a cleaner set of APIs.
>
> The only remaining +1 is for kmalloc_track_caller(), which I ignore what
> it does.
D'oh, of course that's the malloc. Yes, it makes sense to have a +1
there.
>
> char *
> __kmemdup_nul(const char *s, size_t len, gfp_t gfp)
> {
> char *buf;
>
> buf = kmalloc_track_caller(len + 1, gfp);
> if (!buf)
> return NULL;
>
> strcpy(mempcpy(buf, s, len), "");
> return buf;
Alternatively, you can also rewrite the above two lines into one as:
return strncat(strcpy(buf, ""), s, len);
The good thing is that you have strncat() in the kernel, AFAICS.
I reminded myself when checking the definitions that I wrote in shadow:
#define XSTRNDUP(s) \
( \
STRNCAT(strcpy(XMALLOC(strnlen(s, NITEMS(s)) + 1, char), ""), s) \
)
#define STRNDUPA(s) \
( \
STRNCAT(strcpy(alloca(strnlen(s, NITEMS(s)) + 1), ""), s) \
)
Cheers,
Alex
> }
>
> char *
> kstrdup(const char *s, gfp_t gfp)
> {
> return s ? __kmemdup_nul(s, strlen(s), gfp) : NULL;
> }
>
> char *
> kstrndup(const char *s, size_t n, gfp_t gfp)
> {
> return s ? __kmemdup_nul(s, strnlen(s, n), gfp) : NULL;
> }
>
> char *
> kmemdup_nul(const char *s, size_t len, gfp_t gfp)
> {
> return s ? __kmemdup_nul(s, len, gfp) : NULL;
> }
>
> Have a lovely day!
> Alex
>
> --
> <https://www.alejandro-colomar.es/>
--
<https://www.alejandro-colomar.es/>
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