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@xxxxxxxxxxxxxxxxxxxxxxx/ > --- > 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 > >
