perf/core: Replace zero-length array with flexible-array

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200511201227.GA14041@embeddedor

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 87e21681759c0ddb19e37654304b02ed0d018e29..d7b610c4eebd851de82191d18eb8d7325af48872 100644 (file)
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -61,7 +61,7 @@ struct perf_guest_info_callbacks {
 
 struct perf_callchain_entry {
        __u64                           nr;
-       __u64                           ip[0]; /* /proc/sys/kernel/perf_event_max_stack */
+       __u64                           ip[]; /* /proc/sys/kernel/perf_event_max_stack */
 };
 
 struct perf_callchain_entry_ctx {
@@ -113,7 +113,7 @@ struct perf_raw_record {
 struct perf_branch_stack {
        __u64                           nr;
        __u64                           hw_idx;
-       struct perf_branch_entry        entries[0];
+       struct perf_branch_entry        entries[];
 };
 
 struct task_struct;

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index c2b41a263166c72e9c569ab4957aeb928e06dda9..b1991043b7d8495ae293f38dc92226cf67dbd8ac 100644 (file)
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -16,7 +16,7 @@
 
 struct callchain_cpus_entries {
        struct rcu_head                 rcu_head;
-       struct perf_callchain_entry     *cpu_entries[0];
+       struct perf_callchain_entry     *cpu_entries[];
 };
 
 int sysctl_perf_event_max_stack __read_mostly = PERF_MAX_STACK_DEPTH;

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index f16f66b6b655ce5aac7c5635931b0f516fae5b8d..fcbf5616a441185c1669e18ba63a4fa7477ab525 100644 (file)
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -55,7 +55,7 @@ struct perf_buffer {
        void                            *aux_priv;
 
        struct perf_event_mmap_page     *user_page;
-       void                            *data_pages[0];
+       void                            *data_pages[];
 };
 
 extern void rb_free(struct perf_buffer *rb);