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[2a01:4f8:120:2468::2]) by smtp.gmail.com with ESMTPSA id p2-20020a5d4582000000b00203f51aa12asm5208180wrq.55.2022.03.25.07.34.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 25 Mar 2022 07:34:51 -0700 (PDT) From: =?UTF-8?q?=C3=86var=20Arnfj=C3=B6r=C3=B0=20Bjarmason?= To: git@vger.kernel.org Cc: Junio C Hamano , Taylor Blau , =?UTF-8?q?=C3=86var=20Arnfj=C3=B6r=C3=B0=20Bjarmason?= Subject: [PATCH v3] http API: fix dangling pointer issue noted by GCC 12.0 Date: Fri, 25 Mar 2022 15:34:49 +0100 Message-Id: X-Mailer: git-send-email 2.35.1.1509.ge4eeb5bd39e In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org The pre-release GCC 12.0 development branch has a new warning about dangling pointers in -Wall: http.c: In function ‘run_active_slot’: http.c:1332:24: error: storing the address of local variable ‘finished’ in ‘*slot.finished’ [-Werror=dangling-pointer=] 1332 | slot->finished = &finished; | ~~~~~~~~~~~~~~~^~~~~~~~~~~ http.c:1330:13: note: ‘finished’ declared here 1330 | int finished = 0; | ^~~~~~~~ This is on a locally built "gcc (GCC) 12.0.1 20220120 (experimental)", built from gcc.git's 8bc700f4c3f (Enhance vec_pack_trunc for integral mode mask., 2022-01-17). The GCC warning is specifically about pointers that survive the exit of the function. See a comment added to "pass_waccess::use_after_inval_p" in the GCC commit that added the warning, or: /* The use is one of a dangling pointer if a clobber of the variable [the pointer points to] has not been found before the function exit point. */ [...] There's a few possible ways to fix this, but the simplest is to assign NULL to "slot->finished" at the end of run_active_slot(). This isn't the only caller that assigns to "slot->finished", see see the assignments in http-walker.c:process_alternates_response() and http.c:finish_active_slot(). But those assignments are both to the pointer to our local variable here, so this fix is correct. The only way that code in http-walker.c could have done its assignments is to the pointer to this specific variable. It was suggested[2] to guard that with "if (slot->finished == &finished)", but that'll still trigger the warning. 1. https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=9d6a0f388eb048f8d87f47af78f07b5ce513bfe6 2. https://lore.kernel.org/git/xmqq8rv2nggn.fsf@gitster.g/ Signed-off-by: Ævar Arnfjörð Bjarmason --- This should clarify the feedback on v2, sorry about the very late re-roll. I.e. I *meant* in v2 that it's the only assignment to slot->finished itself is here, the other assignments in http-walker.c are to the pointer to our variable here. Range-diff against v2: 1: 777838267a5 ! 1: 69190804c67 http API: fix dangling pointer issue noted by GCC 12.0 @@ Commit message [...] There's a few possible ways to fix this, but the simplest is to assign - NULL to "slot->finished" at the end of run_active_slot(), it's the - only caller that ever assigns non-NULL to it. It was suggested[2] to - guard that with "if (slot->finished == &finished)", but that'll still - trigger the warning. + NULL to "slot->finished" at the end of run_active_slot(). + + This isn't the only caller that assigns to "slot->finished", see see + the assignments in http-walker.c:process_alternates_response() and + http.c:finish_active_slot(). + + But those assignments are both to the pointer to our local variable + here, so this fix is correct. The only way that code in http-walker.c + could have done its assignments is to the pointer to this specific + variable. + + It was suggested[2] to guard that with "if (slot->finished == + &finished)", but that'll still trigger the warning. 1. https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=9d6a0f388eb048f8d87f47af78f07b5ce513bfe6 2. https://lore.kernel.org/git/xmqq8rv2nggn.fsf@gitster.g/ http.c | 1 + 1 file changed, 1 insertion(+) diff --git a/http.c b/http.c index 229da4d1488..2f67fbb33cd 100644 --- a/http.c +++ b/http.c @@ -1367,6 +1367,7 @@ void run_active_slot(struct active_request_slot *slot) select(max_fd+1, &readfds, &writefds, &excfds, &select_timeout); } } + slot->finished = NULL; } static void release_active_slot(struct active_request_slot *slot) -- 2.35.1.1509.ge4eeb5bd39e