Re: [PATCH] trace2: refactor to avoid gcc warning under -O3
From: Johannes Schindelin <hidden>
Date: 2021-05-05 14:39:06
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...
quoted hunk ↗ jump to hunk
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/ (local) --- 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
quoted hunk ↗ jump to hunk
} *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