[PATCH] perf/ring_buffer: Prefer struct_size over open coded arithmetic

[Erick Archer]

This is an effort to get rid of all multiplications from allocation
functions in order to prevent integer overflows [1][2].

As the "rb" variable is a pointer to "struct perf_buffer" and this
structure ends in a flexible array:

struct perf_buffer {
	[...]
	void	*data_pages[];
};

the preferred way in the kernel is to use the struct_size() helper to
do the arithmetic instead of the calculation "size + count * size" in
the kzalloc_node() functions.

In the "rb_alloc" function defined in the else branch of the macro

 #ifndef CONFIG_PERF_USE_VMALLOC

the count in the struct_size helper is the literal "1" since only one
pointer to void is allocated. Also, remove the "size" variable as it
is no longer needed.

This way, the code is more readable and safer.

This code was detected with the help of Coccinelle, and audited and
modified manually.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
Link: https://github.com/KSPP/linux/issues/160 [2]
Signed-off-by: Erick Archer <erick.archer@xxxxxxxxxxx>
---
Hi,

The Coccinelle script used to detect this code pattern is the following:

virtual report

@rule1@
type t1;
type t2;
identifier i0;
identifier i1;
identifier i2;
identifier ALLOC =~ "kmalloc|kzalloc|kmalloc_node|kzalloc_node|vmalloc|vzalloc|kvmalloc|kvzalloc";
position p1;
@@

i0 = sizeof(t1)
..
i0 += sizeof(t2) * i1;
..
i2 = ALLOC@p1(..., i0, ...);

@script:python depends on report@
p1 << rule1.p1;
@@

msg = "WARNING: verify allocation on line %s" % (p1[0].line)
coccilib.report.print_report(p1[0],msg)

Regards,
Erick
---
 kernel/events/ring_buffer.c | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 4013408ce012..e68b02a56382 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -822,9 +822,7 @@ struct perf_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
 	unsigned long size;
 	int i, node;
 
-	size = sizeof(struct perf_buffer);
-	size += nr_pages * sizeof(void *);
-
+	size = struct_size(rb, data_pages, nr_pages);
 	if (order_base_2(size) > PAGE_SHIFT+MAX_PAGE_ORDER)
 		goto fail;
 
@@ -916,15 +914,11 @@ void rb_free(struct perf_buffer *rb)
 struct perf_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
 {
 	struct perf_buffer *rb;
-	unsigned long size;
 	void *all_buf;
 	int node;
 
-	size = sizeof(struct perf_buffer);
-	size += sizeof(void *);
-
 	node = (cpu == -1) ? cpu : cpu_to_node(cpu);
-	rb = kzalloc_node(size, GFP_KERNEL, node);
+	rb = kzalloc_node(struct_size(rb, data_pages, 1), GFP_KERNEL, node);
 	if (!rb)
 		goto fail;
 
-- 
2.25.1