linux-kernel - Re: revert "fs/befs/linuxvfs.c: replace strncpy by strlcpy"

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Date:	Tue, 28 Apr 2015 18:42:10 +0200 (CEST)
From:	Fabian Frederick <fabf@...net.be>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: revert "fs/befs/linuxvfs.c: replace strncpy by strlcpy"



> On 28 April 2015 at 18:05 Al Viro <viro@...IV.linux.org.uk> wrote:
>
>
> On Tue, Apr 28, 2015 at 07:35:10AM +0200, Fabian Frederick wrote:
>
> > > Al, very unhappy about the prospect of looking through ~2000 calls of
> > > strlcpy()
> > > we have in the tree...
> >
> > Sorry Al, I thought it was more secure.
>
> It's not just you, unfortunately, and dumping all that annoyance on you
> as a proxy for everyone who does that kind of thing had been unfair.
> My apologies...

No problem Al :) but why can't we harden strlcpy at first with
something like a strlen limited to max char.
(I don't know if it's already in kernel libs).

size_t strlenl(const char *s, size_t maxlen)
{
        const char *sc = s;
        size_t i = 0;

        while (*sc != '\0' && (i < maxlen)) {
                i++;
                sc++;
        }
        return sc - s;
}

Then we could solve problems downstream ...

Regards,
Fabian

>
> > I guess the 2 following patches should be reversed as well :
> >
> > 6cb103b6f45a
> > "fs/befs/btree.c: replace strncpy by strlcpy + coding style fixing"
> >
> > 69201bb11327
> > "fs/ocfs2/super.c: use OCFS2_MAX_VOL_LABEL_LEN and strlcpy"
>
> AFAICS, they should.
>
> Unfortunately, we _can't_ make strlcpy() never look past src + size - 1 -
> not without changing its semantics.  Its callers expect it to return
> the length of source; one of the intended uses is
>       wanted = strlcpy(dst, src, dst_size);
>       if (wanted >= dst_size) {
>               p = realloc(dst, wanted + 1);
>               if (!p) {
>                       // too bad
>               } else {
>                       dst = p;
>                       memcpy(dst, src, wanted + 1);
>               }
>       }
> and that really wants the accurate length.  Now, the absolute majority of
> strlcpy() users in the kernel completely ignore the return value, i.e. go for
> silent truncation.  About 1% do not.
>
> Out of those, some are correctly implemented "fail if truncated" uses.
> The rest...  Some are weirdly open-coded snprintf() (series of strlcpy and
> strlcat) and some are _very_ dubious.  Either they really never get
> truncated, or we have a problem.  For example, this
> kernel/module.c:2349:                 s += strlcpy(s,
> &mod->strtab[src[i].st_name],
> smells really bad.  We are truncating a bunch of strings dowsn to
> KSYM_NAME_LEN
> there and storing them in a buffer, with spacing that matches _untruncated_
> strings.
>
> drivers/s390/scsi/zfcp_fc.c:825:      len = strlcpy(rspn_req->rspn.fr_name,
> fc_host_symbolic_name(shost),
> also looks fishy - what happens there is
>         len = strlcpy(rspn_req->rspn.fr_name, fc_host_symbolic_name(shost),
>                       FC_SYMBOLIC_NAME_SIZE);
>         rspn_req->rspn.fr_name_len = len;
>
> drivers/usb/gadget/function/f_midi.c:976:     result = strlcpy(page, opts->id,
> PAGE_SIZE);
> drivers/usb/gadget/function/f_printer.c:1172: result = strlcpy(page,
> opts->pnp_string + 2, PNP_STRING_LEN - 2);
> drivers/usb/gadget/function/f_printer.c:1184: result =
> strlcpy(opts->pnp_string + 2, page, PNP_STRING_LEN - 2);
>
> are also strange...
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