Re: [PATCH] trace2: refactor to avoid gcc warning under -O3
From: Junio C Hamano <hidden>
Date: 2021-05-05 09:47:34
Ævar Arnfjörð Bjarmason [off-list ref] 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/20200404142131.GA679473@coredump.intra.peff.net/
---
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.
quoted hunk
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;