Messages in this thread Patch in this message | | | Date | Wed, 20 May 2020 14:16:13 -0500 | From | "Gustavo A. R. Silva" <> | Subject | [PATCH perf/core] perf branch: Replace zero-length array with flexible-array |
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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 and audited _manually_.
[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> --- tools/perf/util/branch.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/tools/perf/util/branch.h b/tools/perf/util/branch.h index 4d3f02fa223df..17b2ccc61094b 100644 --- a/tools/perf/util/branch.h +++ b/tools/perf/util/branch.h @@ -46,7 +46,7 @@ struct branch_entry { struct branch_stack { u64 nr; u64 hw_idx; - struct branch_entry entries[0]; + struct branch_entry entries[]; }; /* -- 2.26.2
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