[PATCH] perf/x86: Replace zero-length array with flexible-array

From: "Gustavo A. R. Silva" <gustavoars@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>,
	"x86@kernel.org H. Peter Anvin" <hpa@zytor.com>
Cc: linux-kernel@vger.kernel.org,
	"Gustavo A. R. Silva" <gustavo@embeddedor.com>
Subject: [PATCH] perf/x86: Replace zero-length array with flexible-array
Date: Mon, 11 May 2020 15:09:11 -0500	[thread overview]
Message-ID: <20200511200911.GA13149@embeddedor> (raw)

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@kernel.org>
---
 arch/x86/events/intel/bts.c    | 2 +-
 arch/x86/events/intel/uncore.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/events/intel/bts.c b/arch/x86/events/intel/bts.c
index 6a3b599ee0fe..731dd8d0dbb1 100644
--- a/arch/x86/events/intel/bts.c
+++ b/arch/x86/events/intel/bts.c
@@ -58,7 +58,7 @@ struct bts_buffer {
 	local_t		head;
 	unsigned long	end;
 	void		**data_pages;
-	struct bts_phys	buf[0];
+	struct bts_phys	buf[];
 };
 
 static struct pmu bts_pmu;
diff --git a/arch/x86/events/intel/uncore.h b/arch/x86/events/intel/uncore.h
index 0da4a4605536..b469ddd45515 100644
--- a/arch/x86/events/intel/uncore.h
+++ b/arch/x86/events/intel/uncore.h
@@ -130,7 +130,7 @@ struct intel_uncore_box {
 	struct list_head list;
 	struct list_head active_list;
 	void __iomem *io_addr;
-	struct intel_uncore_extra_reg shared_regs[0];
+	struct intel_uncore_extra_reg shared_regs[];
 };
 
 /* CFL uncore 8th cbox MSRs */
-- 
2.26.2

