[DTrace-devel] [PATCH] Fix dt_bvar_probedesc() for late USDT processes

[Kris Van Hees]

On Wed, Mar 19, 2025 at 02:32:27AM -0400, eugene.loh at oracle.com wrote:
> From: Eugene Loh <eugene.loh at oracle.com>
> 
> With commit 8bd26415b
> ("bpf: separate bvar implementation into separate functions"),
> test/unittest/usdt/tst.nusdtprobes.sh started failing reproducibly
> on all platforms.
> 
> In that patch, the get_bvar() function is factored into separate
> functions.  It includes a change that looks basically like this:
> 
>     uint32_t    key = mst->prid;
> 
>     if (key < ((uint64_t)&NPROBES)) {
>         [...]
>     } else {
>         char *s = bpf_map_lookup_elem(&usdt_names, &key);
>         switch (idx) {
>   -     case DIF_VAR_PROBENAME: s += DTRACE_FUNCNAMELEN;
>   +     case DIF_VAR_PROBEPROV: s += DTRACE_FUNCNAMELEN;
>   -     case DIF_VAR_PROBEFUNC: s += DTRACE_MODNAMELEN;
>   +     case DIF_VAR_PROBEMOD : s += DTRACE_MODNAMELEN;
>   -     case DIF_VAR_PROBEMOD : s += DTRACE_PROVNAMELEN;
>   +     case DIF_VAR_PROBEFUNC: s += DTRACE_PROVNAMELEN;
>   -     case DIF_VAR_PROBEPROV:
>   +     case DIF_VAR_PROBENAME:
>         }
>         return (uint64_t)s;
>     }
> 
> That is, for the case of key>=NPROBES (that is, for USDT probes that
> were added after the dtrace session was started), the meanings of
> prov, mod, func, and name were exchanged.
> 
> Restore the correct meanings.
> 
> Signed-off-by: Eugene Loh <eugene.loh at oracle.com>

My fault.  Thanks for the fix.
Reviewed-by: Kris Van Hees <kris.van.hees at oracle.com>

> ---
>  bpf/get_bvar.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/bpf/get_bvar.c b/bpf/get_bvar.c
> index c760126da..fadb06c00 100644
> --- a/bpf/get_bvar.c
> +++ b/bpf/get_bvar.c
> @@ -185,13 +185,13 @@ noinline uint64_t dt_bvar_probedesc(const dt_dctx_t *dctx, uint32_t idx)
>  			return (uint64_t)dctx->strtab;
>  
>  		switch (idx) {
> -		case DIF_VAR_PROBEPROV:
> +		case DIF_VAR_PROBENAME:
>  			s += DTRACE_FUNCNAMELEN;
> -		case DIF_VAR_PROBEMOD:
> -			s += DTRACE_MODNAMELEN;
>  		case DIF_VAR_PROBEFUNC:
> +			s += DTRACE_MODNAMELEN;
> +		case DIF_VAR_PROBEMOD:
>  			s += DTRACE_PROVNAMELEN;
> -		case DIF_VAR_PROBENAME:
> +		case DIF_VAR_PROBEPROV:
>  		}
>  
>  		return (uint64_t)s;
> -- 
> 2.43.5
>