linux-kernel - [PATCH -tip V6 6/6] kprobes: Replace zero-length array with flexible-array

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Message-Id: <158927060954.27680.8740399526358942446.stgit@devnote2>
Date:   Tue, 12 May 2020 17:03:29 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Anders Roxell <anders.roxell@...aro.org>, paulmck@...nel.org,
        joel@...lfernandes.org,
        "Naveen N . Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        David Miller <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...e.hu>,
        Peter Zijlstra <peterz@...radead.org>,
        Ziqian SUN <zsun@...hat.com>
Subject: [PATCH -tip V6 6/6] kprobes: Replace zero-length array with flexible-array

From: Gustavo A. R. Silva <gustavoars@...nel.org>

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
Acked-by: Masami Hiramatsu <mhiramat@...nel.org>
---
 include/linux/kprobes.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
index 645fd401c856..53c4f2c1d658 100644
--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h
@@ -161,7 +161,7 @@ struct kretprobe_instance {
 	kprobe_opcode_t *ret_addr;
 	struct task_struct *task;
 	void *fp;
-	char data[0];
+	char data[];
 };

 struct kretprobe_blackpoint {