On Mon, Jun 5, 2017 at 5:25 PM, Tetsuo Handa
<
penguin-kernel@....ne.jp> wrote:
> Hello.
>
> I changed the hook as below and confirmed that cp == NULL at bpf_probe_read().
> That is, it is bpf_usdt_readarg() which got broken.
>
> ----------
> int do_start(struct pt_regs *ctx) {
> char *cp = NULL;
> bpf_usdt_readarg(1, ctx, &cp);
> struct { char query[QUERY_MAX]; } data = { };
> if (cp != NULL)
> bpf_probe_read(&data.query, sizeof(data.query), (void *) cp);
> else
> data.query[0] = '!';
> events.perf_submit(ctx, &data, sizeof(data));
> return 0;
> };
> ----------
>
> If I do
>
> - std::string cptr = tfm::format("*((volatile %s *)dest)", ctype);
> + std::string cptr = tfm::format("*((%s *)dest)", ctype);
>
> (i.e. drop only "volatile " part) in that commit like below
>
> ----------
> diff --git a/src/cc/usdt.cc b/src/cc/usdt.cc
> --- a/src/cc/usdt.cc
> +++ b/src/cc/usdt.cc
> @@ -152,7 +152,7 @@ bool Probe::usdt_getarg(std::
ostream &stream) {
>
> for (size_t arg_n = 0; arg_n < arg_count; ++arg_n) {
> std::string ctype = largest_arg_type(arg_n);
> - std::string cptr("dest");
> + std::string cptr = tfm::format("*((%s *)dest)", ctype);
>
> tfm::format(stream,
> "static __always_inline int _bpf_readarg_%s_%d("
> ----------
>
> USDT probe works again as well as build warning shown below goes away.
>
> ----------
> /virtual/main.c:4:8: warning: incompatible integer to pointer conversion assigning to 'void *' from 'volatile uint64_t' (aka 'volatile unsigned long long') [-Wint-conversion]
> dest = *(volatile uint64_t *)&ctx->bx;
> ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ----------
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