Re: [PATCH] trace2: refactor to avoid gcc warning under -O3
- From
Johannes Schindelin <johannes.schindelin@gmx.de>
- Date
- May 5, 2021, 14:38 UTC
- Message-ID
- <nycvar.QRO.7.76.6.2105051623230.50@tvgsbejvaqbjf.bet>
- In-Reply-To
- <patch-1.1-87d9bcf1095-20210505T083951Z-avarab@gmail.com>
Hi Ævar,
On Wed, 5 May 2021, Ævar Arnfjörð Bjarmason wrote:
Show 13 quoted lines
> 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...
Show 37 quoted lines
>
> 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.
Show 9 quoted lines
> 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
Show 46 quoted lines
> }
>
> *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
>
>