linux-kernel - [PATCH] fcntl: Distribute switch variables for initialization

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Message-Id: <20200220062243.68809-1-keescook@chromium.org>
Date:   Wed, 19 Feb 2020 22:22:43 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] fcntl: Distribute switch variables for initialization

Variables declared in a switch statement before any case statements
cannot be automatically initialized with compiler instrumentation (as
they are not part of any execution flow). With GCC's proposed automatic
stack variable initialization feature, this triggers a warning (and they
don't get initialized). Clang's automatic stack variable initialization
(via CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also
doesn't initialize such variables[1]. Note that these warnings (or silent
skipping) happen before the dead-store elimination optimization phase,
so even when the automatic initializations are later elided in favor of
direct initializations, the warnings remain.

To avoid these problems, move such variables into the "case" where
they're used or lift them up into the main function body.

fs/fcntl.c: In function ‘send_sigio_to_task’:
fs/fcntl.c:738:20: warning: statement will never be executed [-Wswitch-unreachable]
  738 |   kernel_siginfo_t si;
      |                    ^~

[1] https://bugs.llvm.org/show_bug.cgi?id=44916

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/fcntl.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 9bc167562ee8..2e4c0fa2074b 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -735,8 +735,9 @@ static void send_sigio_to_task(struct task_struct *p,
 		return;

 	switch (signum) {
-		kernel_siginfo_t si;
-		default:
+		default: {
+			kernel_siginfo_t si;
+
 			/* Queue a rt signal with the appropriate fd as its
 			   value.  We use SI_SIGIO as the source, not 
 			   SI_KERNEL, since kernel signals always get 
@@ -769,6 +770,7 @@ static void send_sigio_to_task(struct task_struct *p,
 			si.si_fd    = fd;
 			if (!do_send_sig_info(signum, &si, p, type))
 				break;
+		}
 		/* fall-through - fall back on the old plain SIGIO signal */
 		case 0:
 			do_send_sig_info(SIGIO, SEND_SIG_PRIV, p, type);