Re: [PATCH v2] perf tools: avoid sample_reg_masks being const + weak

[Jiri Olsa <jolsa@redhat.com>]

On Fri, Sep 27, 2019 at 02:43:41PM -0700, Ian Rogers wrote:
> Being const + weak breaks with some compilers that constant-propagate
> from the weak symbol. This behavior is outside of the specification, but
> in LLVM is chosen to match GCC's behavior.
> 
> LLVM's implementation was set in this patch:
> https://github.com/llvm/llvm-project/commit/f49573d1eedcf1e44893d5a062ac1b72c8419646
> A const + weak symbol is set to be weak_odr:
> https://llvm.org/docs/LangRef.html
> ODR is one definition rule, and given there is one constant definition
> constant-propagation is possible. It is possible to get this code to
> miscompile with LLVM when applying link time optimization. As compilers
> become more aggressive, this is likely to break in more instances.
> 
> Move the definition of sample_reg_masks to the conditional part of
> perf_regs.h and guard usage with HAVE_PERF_REGS_SUPPORT. This avoids the
> weak symbol.
> 
> Fix an issue when HAVE_PERF_REGS_SUPPORT isn't defined from patch v1.
> 
> Signed-off-by: Ian Rogers <irogers@google.com>

Acked-by: Jiri Olsa <jolsa@kernel.org>

thanks,
jirka

> ---
>  tools/perf/util/parse-regs-options.c | 8 ++++++--
>  tools/perf/util/perf_regs.c          | 4 ----
>  tools/perf/util/perf_regs.h          | 4 ++--
>  3 files changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/tools/perf/util/parse-regs-options.c b/tools/perf/util/parse-regs-options.c
> index ef46c2848808..e687497b3aac 100644
> --- a/tools/perf/util/parse-regs-options.c
> +++ b/tools/perf/util/parse-regs-options.c
> @@ -13,7 +13,7 @@ static int
>  __parse_regs(const struct option *opt, const char *str, int unset, bool intr)
>  {
>  	uint64_t *mode = (uint64_t *)opt->value;
> -	const struct sample_reg *r;
> +	const struct sample_reg *r = NULL;
>  	char *s, *os = NULL, *p;
>  	int ret = -1;
>  	uint64_t mask;
> @@ -46,19 +46,23 @@ __parse_regs(const struct option *opt, const char *str, int unset, bool intr)
>  
>  			if (!strcmp(s, "?")) {
>  				fprintf(stderr, "available registers: ");
> +#ifdef HAVE_PERF_REGS_SUPPORT
>  				for (r = sample_reg_masks; r->name; r++) {
>  					if (r->mask & mask)
>  						fprintf(stderr, "%s ", r->name);
>  				}
> +#endif
>  				fputc('\n', stderr);
>  				/* just printing available regs */
>  				return -1;
>  			}
> +#ifdef HAVE_PERF_REGS_SUPPORT
>  			for (r = sample_reg_masks; r->name; r++) {
>  				if ((r->mask & mask) && !strcasecmp(s, r->name))
>  					break;
>  			}
> -			if (!r->name) {
> +#endif
> +			if (!r || !r->name) {
>  				ui__warning("Unknown register \"%s\", check man page or run \"perf record %s?\"\n",
>  					    s, intr ? "-I" : "--user-regs=");
>  				goto error;
> diff --git a/tools/perf/util/perf_regs.c b/tools/perf/util/perf_regs.c
> index 2774cec1f15f..5ee47ae1509c 100644
> --- a/tools/perf/util/perf_regs.c
> +++ b/tools/perf/util/perf_regs.c
> @@ -3,10 +3,6 @@
>  #include "perf_regs.h"
>  #include "event.h"
>  
> -const struct sample_reg __weak sample_reg_masks[] = {
> -	SMPL_REG_END
> -};
> -
>  int __weak arch_sdt_arg_parse_op(char *old_op __maybe_unused,
>  				 char **new_op __maybe_unused)
>  {
> diff --git a/tools/perf/util/perf_regs.h b/tools/perf/util/perf_regs.h
> index 47fe34e5f7d5..e014c2c038f4 100644
> --- a/tools/perf/util/perf_regs.h
> +++ b/tools/perf/util/perf_regs.h
> @@ -15,8 +15,6 @@ struct sample_reg {
>  #define SMPL_REG2(n, b) { .name = #n, .mask = 3ULL << (b) }
>  #define SMPL_REG_END { .name = NULL }
>  
> -extern const struct sample_reg sample_reg_masks[];
> -
>  enum {
>  	SDT_ARG_VALID = 0,
>  	SDT_ARG_SKIP,
> @@ -27,6 +25,8 @@ uint64_t arch__intr_reg_mask(void);
>  uint64_t arch__user_reg_mask(void);
>  
>  #ifdef HAVE_PERF_REGS_SUPPORT
> +extern const struct sample_reg sample_reg_masks[];
> +
>  #include <perf_regs.h>
>  
>  #define DWARF_MINIMAL_REGS ((1ULL << PERF_REG_IP) | (1ULL << PERF_REG_SP))
> -- 
> 2.23.0.444.g18eeb5a265-goog
>