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From: Al Viro <viro@zeniv.linux.org.uk>
To: Jiri Olsa <jolsa@kernel.org>
Cc: 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: Wed, 29 Jul 2020 21:11:17 +0100
Message-ID: <20200729201117.GA1233513@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20200722211223.1055107-10-jolsa@kernel.org>

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)


> +		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;
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.

  parent 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 [this message]
2020-07-30 10:22     ` Jiri Olsa
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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