linux-kernel - Re: [RFC][PATCH] lib/string: introduce sysfs_strncpy() and sysfs

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Date:   Tue, 21 Aug 2018 18:50:55 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, Martin Wilck <mwilck@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCH] lib/string: introduce sysfs_strncpy() and
 sysfs_strlcpy()

Hi Rasmus,

On (08/21/18 09:59), Rasmus Villemoes wrote:
> > +char *sysfs_strncpy(char *dest, const char *src, size_t count)
> > +{
> > +	char *c;
> > +
> > +	strncpy(dest, skip_spaces(src), count);
> 
> I'd like to see where and how you'd use this, but I'm very skeptical of
> count being used both for the size of the dest buffer as well as an
> essentially random argument to strncpy - if count is also the maximum
> number of bytes to read from the src, you'd need to take the
> skip_spaces() into account, because there are not count bytes left after
> that...
> And if src is not necessarily nul-terminated, skip_spaces() by
> itself is wrong.

I think that sysfs input is always properly NULL-terminated. It may or
may not contain \n, but \0 is expected to be there. Am I wrong?

> Moreover, I don't think we should add more users or wrappers for strncpy
> - I highly doubt the sysfs users you have in mind want the "fill the
> rest of the buffer with '\0'" nor the "not enough room for a terminating
> '\0'? Oh well, what could possibly go wrong" semantics.

The reason I added both strncpy() and strlcpy() was that there are lots
of sysfs ->store() callbacks which use strncpy().

E.g.
	channel_dimm_label_store()
	dimmdev_label_store()
	pmbus_add_label()
	axp20x_store_attr()
	cmdline_store()
	
	and so on and on.

> > +	c = dest + count - 1;
> > +	while (c >= dest && (isspace(*c) || *c == '\n' || *c == '\0')) {
> 
> nit: '\n' certainly already passes the isspace() test.

Hah, indeed, it should.
"\n" & 0x20 must be positive. Andrew had the same comment. But I didn't
check what actually isspace() was doing, until now.

> > +size_t sysfs_strlcpy(char *dest, const char *src, size_t size)
> > +{
> > +	size_t ret;
> > +	char *c;
> > +
> > +	ret = strlcpy(dest, skip_spaces(src), size);
> > +
> > +	size = strlen(dest);
> > +	c = dest + size - 1;
> > +	while (c >= dest && (isspace(*c) || *c == '\n'))
> > +		c--;
> > +	*(c + 1) = '\0';
> > +	return ret;
> > +}
> 
> What exactly is the return value?

Honestly, I didn't think about it. I wasn't sure if we want to return
anything from this function and from sysfs_strncpy(). I glanced through
a number of ->store() callbacks, and it seems that mostly people don't
bother to check strlcpy() return value at all.

> A more useful return value would either be "the length of the string
> now in dest", or some sort of indicator that the input was truncated,
> if that is ever possible.

Agreed.

> I think you're too focused on making wrappers around str[ln]cpy
> preserving parts of those functions' API. Instead, try to figure out
> what sysfs users actually want, name the functions after that, and then
> whether they use strncpy or sprintf or strscpy internally is completely
> irrelevant.

Going point, that's why the patch is in RFC stage: to figure out
what do we actually want.

> int strcpy_trim(char *dst, size_t dstsize, const char *src, size_t
> srcsize) - copy (potentially not '\0'-terminated) src to dst, trimming
> leading and trailing whitespace. dstsize must be positive, and dst is
> guaranteed to be '\0'-terminated. Returns the length of the string now
> in dst, or -EOVERFLOW if some none-whitespace character was chopped.
>
> would cover all use cases?

I like it in general. Sounds like a plan to me. Maybe the "-EOVERFLOW if
some none-whitespace character was chopped" part can be changed: if we
would trim leading and trailing whitespaces before we copy a string then
only valid input chars can get chopped.

	-ss