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

From: Junio C Hamano <[email-masked]>
Date: Wed, 05 May 2021 18:47:29 +0900

Ævar Arnfjörð Bjarmason  <[email-masked]> writes:

> 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.
>
> 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/[email-masked]/
> ---
>  trace2/tr2_dst.c | 27 +++++++++++++++------------
>  1 file changed, 15 insertions(+), 12 deletions(-)

The patch makes sense to me, modulo that the way the variable
"saved_errno" introduced by this patch is used and the way a
variable with that name is typically used in our codebase are at
odds.  I.e. we typically call a variable "saved_errno" when it is
used in this pattern:

    if (a_syscall_whose_error_condition_we_care_about()) {
	int saved_errno = errno;
	perform_some_cleanup_operation_that_might_clobber_errno();
	return error_errno(..., saved_errno);
	/*
	 * or
	 * errno = saved_errno;
	 * return -1;
	 * and let the caller handle 'errno'
	 */
    }

But since I do not think of a better name for this new variable that
is not exactly used like so, let's queue it as-is.

Thanks.

> 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;
> +	}
>  
>  	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;
>  	}
>  
>  	*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;