Re: [PATCH] gcov: fix clang-11+ support

[Fangrui Song]

On 2021-03-12, Nick Desaulniers wrote:
> LLVM changed the expected function signatures for llvm_gcda_start_file()
> and llvm_gcda_emit_function() in the clang-11 release. Users of clang-11
> or newer may have noticed their kernels failing to boot due to a panic
> when enabling CONFIG_GCOV_KERNEL=y +CONFIG_GCOV_PROFILE_ALL=y.  Fix up
> the function signatures so calling these functions doesn't panic the
> kernel.
>
> When we drop clang-10 support from the kernel, we should carefully
> update the original implementations to try to preserve git blame,
> deleting these implementations.
>
> Link: https://reviews.llvm.org/rGcdd683b516d147925212724b09ec6fb792a40041
> Link: https://reviews.llvm.org/rG13a633b438b6500ecad9e4f936ebadf3411d0f44
> Cc: Fangrui Song <maskray@xxxxxxxxxx>
> Reported-by: Prasad Sodagudi<psodagud@xxxxxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> ---
> kernel/gcov/clang.c | 69 +++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 69 insertions(+)
>
> diff --git a/kernel/gcov/clang.c b/kernel/gcov/clang.c
> index c94b820a1b62..20e6760ec05d 100644
> --- a/kernel/gcov/clang.c
> +++ b/kernel/gcov/clang.c
> @@ -75,7 +75,9 @@ struct gcov_fn_info {
>
> 	u32 num_counters;
> 	u64 *counters;
> +#if __clang_major__ < 11
> 	const char *function_name;
> +#endif

function_name can be unconditionally deleted. It is not used by llvm-cov
gcov.  You'll need to delete a few assignments to gcov_info_free but you
can then unify the gcov_fn_info_dup and gcov_info_free implementations.

> };
>
> static struct gcov_info *current_info;
> @@ -105,6 +107,7 @@ void llvm_gcov_init(llvm_gcov_callback writeout, llvm_gcov_callback flush)
> }
> EXPORT_SYMBOL(llvm_gcov_init);
>
> +#if __clang_major__ < 11
> void llvm_gcda_start_file(const char *orig_filename, const char version[4],
> 		u32 checksum)
> {
> @@ -113,7 +116,17 @@ void llvm_gcda_start_file(const char *orig_filename, const char version[4],
> 	current_info->checksum = checksum;
> }
> EXPORT_SYMBOL(llvm_gcda_start_file);
> +#else
> +void llvm_gcda_start_file(const char *orig_filename, u32 version, u32 checksum)
> +{
> +	current_info->filename = orig_filename;
> +	current_info->version = version;
> +	current_info->checksum = checksum;
> +}
> +EXPORT_SYMBOL(llvm_gcda_start_file);
> +#endif

LG. On big-endian systems, clang < 11 emitted .gcno/.gcda files do not
work with llvm-cov gcov < 11.  To fix it and make .gcno/.gcda work with
gcc gcov I chose to break compatibility (and make all the breaking
changes like deleting some CC1 options) in a short window. At that time
I was not aware that there is the kernel implementation. Later on I was
CCed on a few https://github.com/ClangBuiltLinux/linux/ gcov issues but
I forgot to mention the interface change.

Now in clang 11 onward, clang --coverage defaults to the gcov 4.8
compatible format. You can specify the CC1 option (internal option,
subject to change) -coverage-version to make it compatible with other
versions' gcov.

-Xclang -coverage-version='407*' => 4.7
-Xclang -coverage-version='704*' => 7.4
-Xclang -coverage-version='B02*' => 10.2 (('B'-'A')*10 = 10)

Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>

> +#if __clang_major__ < 11
> void llvm_gcda_emit_function(u32 ident, const char *function_name,
> 		u32 func_checksum, u8 use_extra_checksum, u32 cfg_checksum)
> {
> @@ -133,6 +146,24 @@ void llvm_gcda_emit_function(u32 ident, const char *function_name,
> 	list_add_tail(&info->head, &current_info->functions);
> }
> EXPORT_SYMBOL(llvm_gcda_emit_function);
> +#else
> +void llvm_gcda_emit_function(u32 ident, u32 func_checksum,
> +		u8 use_extra_checksum, u32 cfg_checksum)
> +{
> +	struct gcov_fn_info *info = kzalloc(sizeof(*info), GFP_KERNEL);
> +
> +	if (!info)
> +		return;
> +
> +	INIT_LIST_HEAD(&info->head);
> +	info->ident = ident;
> +	info->checksum = func_checksum;
> +	info->use_extra_checksum = use_extra_checksum;
> +	info->cfg_checksum = cfg_checksum;
> +	list_add_tail(&info->head, &current_info->functions);
> +}
> +EXPORT_SYMBOL(llvm_gcda_emit_function);
> +#endif
>
> void llvm_gcda_emit_arcs(u32 num_counters, u64 *counters)
> {
> @@ -295,6 +326,7 @@ void gcov_info_add(struct gcov_info *dst, struct gcov_info *src)
> 	}
> }
>
> +#if __clang_major__ < 11
> static struct gcov_fn_info *gcov_fn_info_dup(struct gcov_fn_info *fn)
> {
> 	size_t cv_size; /* counter values size */
> @@ -322,6 +354,28 @@ static struct gcov_fn_info *gcov_fn_info_dup(struct gcov_fn_info *fn)
> 	kfree(fn_dup);
> 	return NULL;
> }
> +#else
> +static struct gcov_fn_info *gcov_fn_info_dup(struct gcov_fn_info *fn)
> +{
> +	size_t cv_size; /* counter values size */
> +	struct gcov_fn_info *fn_dup = kmemdup(fn, sizeof(*fn),
> +			GFP_KERNEL);
> +	if (!fn_dup)
> +		return NULL;
> +	INIT_LIST_HEAD(&fn_dup->head);
> +
> +	cv_size = fn->num_counters * sizeof(fn->counters[0]);
> +	fn_dup->counters = vmalloc(cv_size);
> +	if (!fn_dup->counters) {
> +		kfree(fn_dup);
> +		return NULL;
> +	}
> +
> +	memcpy(fn_dup->counters, fn->counters, cv_size);
> +
> +	return fn_dup;
> +}
> +#endif
>
> /**
>  * gcov_info_dup - duplicate profiling data set
> @@ -362,6 +416,7 @@ struct gcov_info *gcov_info_dup(struct gcov_info *info)
>  * gcov_info_free - release memory for profiling data set duplicate
>  * @info: profiling data set duplicate to free
>  */
> +#if __clang_major__ < 11
> void gcov_info_free(struct gcov_info *info)
> {
> 	struct gcov_fn_info *fn, *tmp;
> @@ -375,6 +430,20 @@ void gcov_info_free(struct gcov_info *info)
> 	kfree(info->filename);
> 	kfree(info);
> }
> +#else
> +void gcov_info_free(struct gcov_info *info)
> +{
> +	struct gcov_fn_info *fn, *tmp;
> +
> +	list_for_each_entry_safe(fn, tmp, &info->functions, head) {
> +		vfree(fn->counters);
> +		list_del(&fn->head);
> +		kfree(fn);
> +	}
> +	kfree(info->filename);
> +	kfree(info);
> +}
> +#endif
>
> #define ITER_STRIDE	PAGE_SIZE
>
>
> base-commit: f78d76e72a4671ea52d12752d92077788b4f5d50
> --
> 2.31.0.rc2.261.g7f71774620-goog