Re: PATCH v7] tracing/eprobes: Allow use of BTF names to dereference pointers

[Steven Rostedt]

On Sat, 30 May 2026 23:14:27 +0900
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> wrote:

> Thanks, and Sashiko reviewed this. 
> 
> https://sashiko.dev/#/patchset/20260529110442.0967a64c%40fedora
> 
> I think both comments are valid, especially, second one is important.

Hmm, comment below.

> 
> BTW, I updated probes/fixes. Could you also update this and rebase
> on probes/fixes branch?
> 
> I'm working on the nesting and container_of support patches which
> are on top of this.

OK, I'll make sure to use your probes branch. Which one should I use? your
probes/for-next or probes/urgent?

As for sashiko's comments:

> commit c8393ea9936934c16ba2881d9e7b3cce86c678ff
> Author: Steven Rostedt <rostedt@xxxxxxxxxxx>
> [PATCH v7] tracing/eprobes: Allow use of BTF names to dereference pointers
> This commit introduces syntax to allow event probes to typecast pointer
> fields to a structure using BTF. This enables dereferencing structures
> using BTF names rather than having to manually calculate offsets.
> > diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
> > index 39f040c863e85..0e87409a44033 100644
> > --- a/kernel/trace/trace_probe.c
> > +++ b/kernel/trace/trace_probe.c
> [ ... ]
> > @@ -640,7 +673,7 @@ static int parse_btf_arg(char *varname,
> >  	int i, is_ptr, ret;
> >  	u32 tid;
> >  
> > -	if (WARN_ON_ONCE(!ctx->funcname))
> > +	if (WARN_ON_ONCE(!ctx->funcname && !(ctx->flags & TPARG_FL_TEVENT)))
> >  		return -EINVAL;
> >  
> >  	is_ptr = split_next_field(varname, &field, ctx);
> > @@ -653,6 +686,16 @@ static int parse_btf_arg(char *varname,
> >  		return -EOPNOTSUPP;
> >  	}
> >  
> > +	if (ctx->flags & TPARG_FL_TEVENT) {
> > +		ret = parse_trace_event(varname, code, ctx);
> > +		if (ret < 0)
> > +			return ret;

> Does this silently drop the error on parsing failure?
> If parse_trace_event() fails and returns -EINVAL, it appears this path
> returns the error without logging it.
> Unlike parse_probe_vars(), which explicitly logs the failure when
> parse_trace_event() fails, this path bypasses the log. Could this make
> it difficult to debug invalid typecast expressions since nothing is written
> to tracing/error_log?

Hmm, I guess I can put an error log here, as this is a unique path.

> > +		if (WARN_ON_ONCE(!(ctx->flags & TPARG_FL_TYPECAST)))
> > +			return -EINVAL;
> > +		type = ctx->last_struct;
> > +		goto found_type;
> > +	}
> > +
> >  	if (ctx->flags & TPARG_FL_RETURN && !strcmp(varname, "$retval")) {
> >  		code->op = FETCH_OP_RETVAL;
> [ ... ]
> > @@ -758,7 +802,71 @@ static int parse_btf_bitfield(struct fetch_insn **pcode,
> >  	return 0;
> >  }
> >  
> > -#else
> > +static int query_btf_struct(const char *sname, struct traceprobe_parse_context *ctx)
> [ ... ]
> > +static int handle_typecast(char *arg, struct fetch_insn **pcode,
> > +			   struct fetch_insn *end,
> > +			   struct traceprobe_parse_context *ctx)
> > +{
> > +	char *tmp;
> > +	int ret;
> > +
> > +	/* Currently this only works for eprobes */
> > +	if (!(ctx->flags & TPARG_FL_TEVENT)) {
> > +		trace_probe_log_err(ctx->offset, TYPECAST_NOT_EVENT);
> > +		return -EINVAL;
> > +	}
> > +
> > +	tmp = strchr(arg, ')');
> > +	if (!tmp) {
> > +		trace_probe_log_err(ctx->offset + strlen(arg),
> > +				    DEREF_OPEN_BRACE);
> > +		return -EINVAL;
> > +	}
> > +	*tmp = '\0';
> > +	ret = query_btf_struct(arg + 1, ctx);
> > +	*tmp = ')';
> > +
> > +	if (ret < 0) {
> > +		trace_probe_log_err(ctx->offset + 1, NO_PTR_STRCT);
> > +		ret = -EINVAL;
> > +		goto out_put;
> > +	}
> > +
> > +	ctx->flags |= TPARG_FL_TYPECAST;
> > +	tmp++;
> > +
> > +	ctx->offset += tmp - arg;
> > +	ret = parse_btf_arg(tmp, pcode, end, ctx);
> > +	ctx->flags &= ~TPARG_FL_TYPECAST;
> > +	ctx->last_struct = NULL;
> > +out_put:
> > +	btf_put(ctx->struct_btf);

> Does this prematurely release the BTF struct reference?
> If TPARG_FL_TYPECAST is unset here and ctx->struct_btf is put, won't
> later steps in traceprobe_parse_probe_arg_body() (like
> find_fetch_type_from_btf_type()) fail to properly infer struct field sizes?
> When ctx_btf(ctx) is called later without TPARG_FL_TYPECAST set, it
> will evaluate to ctx->btf (which is NULL for eprobes).
> Could this potentially lead to silent defaults, such as 64-bit reads for
> smaller fields, or fail to inject pointer dereferences for string fields,
> while also leaving ctx->last_type pointing to a prematurely released BTF
> object?

Does this mean we need to set ctx->last_type to NULL here too?

Because everything above is pretty much the expected behavior. The put is
*not* premature. The last_struct and struct_btf are both set to NULL. I
guess the only thing missing is to reset last_type as well.

-- Steve


> > +	ctx->struct_btf = NULL;
> > +	return ret;
> > +}
> > +
> > +#else /* !CONFIG_PROBE_EVENTS_BTF_ARGS */