Re: [PATCH] trace2: refactor to avoid gcc warning under -O3

From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
Cc: git@vger.kernel.org, Junio C Hamano <gitster@pobox.com>,
	Jeff King <peff@peff.net>, Denton Liu <liu.denton@gmail.com>,
	Jeff Hostetler <jeffhost@microsoft.com>
Subject: Re: [PATCH] trace2: refactor to avoid gcc warning under -O3
Date: Wed, 5 May 2021 16:38:42 +0200 (CEST)	[thread overview]
Message-ID: <nycvar.QRO.7.76.6.2105051623230.50@tvgsbejvaqbjf.bet> (raw)
In-Reply-To: <patch-1.1-87d9bcf1095-20210505T083951Z-avarab@gmail.com>

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Hi Ævar,

On Wed, 5 May 2021, Ævar Arnfjörð Bjarmason wrote:

> Refactor tr2_dst_try_uds_connect() to avoid a gcc warning[1] that
> appears under -O3 (but not -O2). This makes the build pass under
> DEVELOPER=1 without needing a DEVOPTS=no-error.
>
> This can be reproduced with GCC Debian 8.3.0-6, but not e.g. with
> clang 7.0.1-8+deb10u2. We've had this warning since
> ee4512ed481 (trace2: create new combined trace facility, 2019-02-22).
>
> As noted in [2] this warning happens because the compiler doesn't
> assume that errno must be non-zero after a failed syscall. Let's work
> around it as suggested in that analysis. We now return -1 ourselves on
> error, and save away the value of errno in a variable the caller
> passes in.

It would probably be a lot nicer if you lead with this insight. I could
imagine, for example, that a oneline like this would be much more helpful
to any reader:

	trace2: do not assume errno != 0 after a failed syscall

The first two paragraphs are less interesting than the third paragraph,
too, therefore I would recommend

About the patch...

>
> 1.
>
>     trace2/tr2_dst.c: In function ‘tr2_dst_get_trace_fd.part.5’:
>     trace2/tr2_dst.c:296:10: warning: ‘fd’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>       dst->fd = fd;
>       ~~~~~~~~^~~~
>     trace2/tr2_dst.c:229:6: note: ‘fd’ was declared here
>       int fd;
>           ^~
> 2. https://lore.kernel.org/git/20200404142131.GA679473@coredump.intra.peff.net/
> ---
>  trace2/tr2_dst.c | 27 +++++++++++++++------------
>  1 file changed, 15 insertions(+), 12 deletions(-)
>
> diff --git a/trace2/tr2_dst.c b/trace2/tr2_dst.c
> index ae052a07fe2..c2aba71041b 100644
> --- a/trace2/tr2_dst.c
> +++ b/trace2/tr2_dst.c
> @@ -197,22 +197,25 @@ static int tr2_dst_try_path(struct tr2_dst *dst, const char *tgt_value)
>  #define PREFIX_AF_UNIX_STREAM "af_unix:stream:"
>  #define PREFIX_AF_UNIX_DGRAM "af_unix:dgram:"
>
> -static int tr2_dst_try_uds_connect(const char *path, int sock_type, int *out_fd)
> +static int tr2_dst_try_uds_connect(const char *path, int sock_type,
> +				   int *out_fd, int *saved_errno)
>  {
>  	int fd;
>  	struct sockaddr_un sa;
>
>  	fd = socket(AF_UNIX, sock_type, 0);
> -	if (fd == -1)
> -		return errno;
> +	if (fd == -1) {
> +		*saved_errno = errno;
> +		return -1;
> +	}

I don't think this is necessary. My manual page for socket(2) says this:

	RETURN VALUE
		If the connection or binding succeeds, zero is returned.
		On error, -1 is returned, and errno is set appropriately.

>  	sa.sun_family = AF_UNIX;
>  	strlcpy(sa.sun_path, path, sizeof(sa.sun_path));
>
>  	if (connect(fd, (struct sockaddr *)&sa, sizeof(sa)) == -1) {
> -		int e = errno;
> +		*saved_errno = errno;
>  		close(fd);
> -		return e;
> +		return -1;

Likewise, my manual page for connect(2) says the same as for socket(2):
upon return value -1, errno is set appropriately (i.e. non-zero).

Therefore, I would say this patch is actually only papering over an
overzealous (and incorrect) compiler warning.

If you _must_ shut up GCC, a better idea would be a much less intrusive,
easier to read

		/* GCC thinks socket()/connect() might fail to set errno */
		return errno ? errno : EIO;

Ciao,
Dscho

>  	}
>
>  	*out_fd = fd;
> @@ -227,7 +230,7 @@ static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
>  {
>  	unsigned int uds_try = 0;
>  	int fd;
> -	int e;
> +	int saved_errno;
>  	const char *path = NULL;
>
>  	/*
> @@ -271,15 +274,15 @@ static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
>  	}
>
>  	if (uds_try & TR2_DST_UDS_TRY_STREAM) {
> -		e = tr2_dst_try_uds_connect(path, SOCK_STREAM, &fd);
> -		if (!e)
> +		if (!tr2_dst_try_uds_connect(path, SOCK_STREAM, &fd,
> +					     &saved_errno))
>  			goto connected;
> -		if (e != EPROTOTYPE)
> +		if (saved_errno != EPROTOTYPE)
>  			goto error;
>  	}
>  	if (uds_try & TR2_DST_UDS_TRY_DGRAM) {
> -		e = tr2_dst_try_uds_connect(path, SOCK_DGRAM, &fd);
> -		if (!e)
> +		if (!tr2_dst_try_uds_connect(path, SOCK_DGRAM, &fd,
> +					     &saved_errno))
>  			goto connected;
>  	}
>
> @@ -287,7 +290,7 @@ static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
>  	if (tr2_dst_want_warning())
>  		warning("trace2: could not connect to socket '%s' for '%s' tracing: %s",
>  			path, tr2_sysenv_display_name(dst->sysenv_var),
> -			strerror(e));
> +			strerror(saved_errno));
>
>  	tr2_dst_trace_disable(dst);
>  	return 0;
> --
> 2.31.1.745.g2af7c6593ce
>
>