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From: Jiri Olsa <jolsa@redhat.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jiri Olsa <jolsa@kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andriin@fb.com>,
	netdev@vger.kernel.org, bpf@vger.kernel.org,
	Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
	Martin KaFai Lau <kafai@fb.com>, David Miller <davem@redhat.com>,
	John Fastabend <john.fastabend@gmail.com>,
	Wenbo Zhang <ethercflow@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	Brendan Gregg <bgregg@netflix.com>,
	Florent Revest <revest@chromium.org>
Subject: Re: [PATCH v8 bpf-next 09/13] bpf: Add d_path helper
Date: Thu, 30 Jul 2020 12:22:52 +0200
Message-ID: <20200730102252.GR1319041@krava> (raw)
In-Reply-To: <20200729201117.GA1233513@ZenIV.linux.org.uk>

On Wed, Jul 29, 2020 at 09:11:17PM +0100, Al Viro wrote:
> On Wed, Jul 22, 2020 at 11:12:19PM +0200, Jiri Olsa wrote:
> 
> > +BPF_CALL_3(bpf_d_path, struct path *, path, char *, buf, u32, sz)
> > +{
> > +	char *p = d_path(path, buf, sz - 1);
> > +	int len;
> > +
> > +	if (IS_ERR(p)) {
> > +		len = PTR_ERR(p);
> > +	} else {
> > +		len = strlen(p);
> > +		if (len && p != buf)
> > +			memmove(buf, p, len);
> 
> *blink*
> What the hell do you need that strlen() for?  d_path() copies into
> the end of buffer (well, starts there and prepends to it); all you
> really need is memmove(buf, p, buf + sz - p)

I used the code from some of the other users like
  backing_dev_show
  fsg_show_file

nice, looks like we could omit strlen call in perf mmap event call as well

> 
> 
> > +		buf[len] = 0;
> 
> Wait a minute...  Why are you NUL-terminating it separately?
> You do rely upon having NUL in the damn thing (and d_path() does
> guarantee it there).  Without that strlen() would've gone into
> the nasal demon country; you can't call it on non-NUL-terminated
> array.  So you are guaranteed that p[len] will be '\0'; why bother
> copying the first len bytes and then separately deal with that
> NUL?  Just memmove() the fucker and be done with that...
> 
> If you are worried about stray NUL in the middle of the returned
> data... can't happen.  Note the rename_lock use in fs/d_path.c;
> the names of everything involved are guaranteed to have been
> stable throughout the copying them into the buffer - if anything
> were to be renamed while we are doing that, we'd repeat the whole
> thing (with rename_lock taken exclusive the second time around).
> 
> So make it simply
> 	if (IS_ERR(p))
> 		return PTR_ERR(p);
> 	len = buf + sz - p;
> 	memmove(buf, p, len);
> 	return len;

ok, will use this

> and be done with that.  BTW, the odds of p == buf are pretty much
> nil - it would happen only if sz - 1 happened to be the exact length
> of pathname.
> 

ok, great

thanks,
jirka


  reply index

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-22 21:12 [PATCH v8 bpf-next 00/13] " Jiri Olsa
2020-07-22 21:12 ` [PATCH v8 bpf-next 01/13] selftests/bpf: Fix resolve_btfids test Jiri Olsa
2020-07-28 20:27   ` Andrii Nakryiko
2020-07-29 20:06     ` Jiri Olsa
2020-07-29 21:38       ` Andrii Nakryiko
2020-07-22 21:12 ` [PATCH v8 bpf-next 02/13] tools resolve_btfids: Add support for set symbols Jiri Olsa
2020-07-28  0:53   ` Andrii Nakryiko
2020-07-28  9:35     ` Jiri Olsa
2020-07-22 21:12 ` [PATCH v8 bpf-next 03/13] bpf: Move btf_resolve_size into __btf_resolve_size Jiri Olsa
2020-07-28 20:29   ` Andrii Nakryiko
2020-07-22 21:12 ` [PATCH v8 bpf-next 04/13] bpf: Add elem_id pointer as argument to __btf_resolve_size Jiri Olsa
2020-07-28 20:30   ` Andrii Nakryiko
2020-07-22 21:12 ` [PATCH v8 bpf-next 05/13] bpf: Add type_id " Jiri Olsa
2020-07-22 21:12 ` [PATCH v8 bpf-next 06/13] bpf: Factor btf_struct_access function Jiri Olsa
2020-07-28 23:27   ` Andrii Nakryiko
2020-07-29 15:59     ` Jiri Olsa
2020-07-22 21:12 ` [PATCH v8 bpf-next 07/13] bpf: Add btf_struct_ids_match function Jiri Olsa
2020-07-28 23:35   ` Andrii Nakryiko
2020-07-29 16:04     ` Jiri Olsa
2020-07-29 17:51       ` Andrii Nakryiko
2020-07-29 18:55         ` Jiri Olsa
2020-07-22 21:12 ` [PATCH v8 bpf-next 08/13] bpf: Add BTF_SET_START/END macros Jiri Olsa
2020-07-28 19:39   ` Andrii Nakryiko
2020-07-29 11:54     ` Jiri Olsa
2020-07-22 21:12 ` [PATCH v8 bpf-next 09/13] bpf: Add d_path helper Jiri Olsa
2020-07-28 19:47   ` Andrii Nakryiko
2020-07-29 15:54     ` Jiri Olsa
2020-07-29 20:11   ` Al Viro
2020-07-30 10:22     ` Jiri Olsa [this message]
2020-07-29 20:17   ` Al Viro
2020-07-30 10:09     ` Jiri Olsa
2020-07-22 21:12 ` [PATCH v8 bpf-next 10/13] bpf: Update .BTF_ids section in btf.rst with sets info Jiri Olsa
2020-07-22 21:12 ` [PATCH v8 bpf-next 11/13] selftests/bpf: Add verifier test for d_path helper Jiri Olsa
2020-07-28 19:49   ` Andrii Nakryiko
2020-07-22 21:12 ` [PATCH v8 bpf-next 12/13] selftests/bpf: Add " Jiri Olsa
2020-07-28 19:53   ` Andrii Nakryiko
2020-07-29 11:25     ` Jiri Olsa
2020-07-22 21:12 ` [PATCH v8 bpf-next 13/13] selftests/bpf: Add set test to resolve_btfids Jiri Olsa
2020-07-28 19:56   ` Andrii Nakryiko
2020-07-29 15:34     ` Jiri Olsa

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