Re: [PATCH v3 bpf-next 09/11] bpftool: Add ability to dump LOC_PARAM, LOC_PROTO and LOCSEC

[Quentin Monnet <qmo@kernel.org>]

2026-09-16 08:41 UTC+0100 ~ Alan Maguire <alan.maguire@oracle.com>
> In raw mode ensure we can dump new BTF kinds in normal/json format.
> BTF_KIND_LOC_PARAMs are rendered as strings, for example a
> const value of 0x2a and a dereference of r10 + 0x20:
> 
>   [12] LOC_PARAM '(anon)' size=4 flags=0x2 vlen=1 values='0x2a'
>   [13] LOC_PARAM '(anon)' size=8 flags=0x38 vlen=2 values='*(r10 + 0x20)'
> 
> LOC_PROTOs render the associated values of each of their
> LOC_PARAMs for easier readability:
> 
>   [14] LOC_PROTO '(anon)' vlen=2
>   	type_id=12 value='r1'
>   	type_id=13 value='*(r2 + 0x10)'
> 
> and LOCSEC shows function name associated with site:
> 
>   [15] LOCSEC 'inline.text' vlen=1
>   	name=foo func_type_id=5 loc_proto_type_id=14 offset=64
> 
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> ---
>  tools/bpf/bpftool/btf.c | 169 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 169 insertions(+)
> 
> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> index bbe8f9ea144f..5e0cb5862811 100644
> --- a/tools/bpf/bpftool/btf.c
> +++ b/tools/bpf/bpftool/btf.c
> @@ -51,6 +51,9 @@ static const char * const btf_kind_str[NR_BTF_KINDS] = {
>  	[BTF_KIND_DECL_TAG]	= "DECL_TAG",
>  	[BTF_KIND_TYPE_TAG]	= "TYPE_TAG",
>  	[BTF_KIND_ENUM64]	= "ENUM64",
> +	[BTF_KIND_LOC_PARAM]	= "LOC_PARAM",
> +	[BTF_KIND_LOC_PROTO]	= "LOC_PROTO",
> +	[BTF_KIND_LOCSEC]	= "LOCSEC",
>  };
>  
>  struct sort_datum {
> @@ -117,6 +120,83 @@ static int btf_kind_safe(int kind)
>  	return kind <= BTF_KIND_MAX ? kind : BTF_KIND_UNKN;
>  }
>  
> +static void btf_loc_param_str(const struct btf_type *t, char *str, size_t sz)
> +{
> +	const struct btf_loc_param *p;
> +	__u32 i = 0, vlen;
> +	__u64 value;
> +	bool negative = false;
> +	char regs[32] = {};
> +	char num[32] = {};
> +	const char *op = "";
> +
> +	if (!t || !btf_is_loc_param(t)) {
> +		snprintf(str, sz, "<invalid>");
> +		return;
> +	}
> +
> +	p = btf_loc_param(t);
> +	vlen = btf_vlen(t);
> +
> +	if (p->flags & BTF_LOC_PARAM_REG) {
> +		__u32 nregs = (p->flags == BTF_LOC_PARAM_REG) ? vlen : 1;
> +
> +		if (nregs > vlen) {
> +			snprintf(str, sz, "?");
> +			return;
> +		}
> +
> +		switch (nregs) {
> +		case 2:
> +			snprintf(regs, sizeof(regs), "r%u, r%u",
> +				 p->values[0], p->values[1]);
> +			break;
> +		case 1:
> +			snprintf(regs, sizeof(regs), "r%u", p->values[0]);
> +			break;
> +		default:
> +			snprintf(regs, sizeof(regs), "?");
> +			break;
> +		}
> +		i += nregs;
> +	}
> +	if (p->flags & (BTF_LOC_PARAM_CONST|BTF_LOC_PARAM_OFFSET)) {
> +		switch (vlen - i) {
> +		case 1:
> +			value = p->values[i];
> +			break;
> +		case 2:
> +			value = ((__u64)p->values[i + 1] << 32) | p->values[i];
> +			break;
> +		default:
> +			snprintf(num, sizeof(num), "?");
> +			goto done;
> +		}
> +		if ((p->flags & BTF_LOC_PARAM_SIGNED) && t->size &&
> +		    t->size <= sizeof(value)) {
> +			__u32 bits = t->size * 8;


Hi Alan, thanks!

In the case of BTF_LOC_PARAM_OFFSET, what do we need "negative" for
exactly, is this supposed to be the sign for the parameter or for the
associated offset? My understanding is that "t->size" refers to the
parameter itself, not the offset, so we would pick the wrong bit if
trying to find the sign for the offset? But I'm not sure I read it
correctly.


> +
> +			if (t->size < sizeof(value))
> +				value &= (1ULL << bits) - 1;
> +			negative = value & (1ULL << (bits - 1));
> +			if (negative)
> +				value = t->size == sizeof(value) ? -value :
> +					(1ULL << bits) - value;
> +		}
> +		snprintf(num, sizeof(num), "0x%llx", (unsigned long long)value);


Looking at the different existing flags and their docs, I see:

"a BTF_LOC_PARAM_ADDR|BTF_LOC_PARAM_CONST is an address that should be
normalized with respect to kernel/module base address."

But I don't see the output accounting for BTF_LOC_PARAM_ADDR, is this
expected or is that an omission?

[...]

Please also look at Sashiko and bpf-ci's reviews for patch 7.

Thanks,
Quentin