linux-kernel - Re: [PATCH bpf-next v3 3/6] resolve_btfids: Introduce enum btf_id

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Message-ID: <c857acb9-977a-49ca-a03f-ef3fd68fabae@linux.dev>
Date: Mon, 15 Dec 2025 18:31:02 -0800
From: Ihor Solodrai <ihor.solodrai@...ux.dev>
To: Eduard Zingerman <eddyz87@...il.com>, Alexei Starovoitov
 <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
 Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nsc@...nel.org>,
 Tejun Heo <tj@...nel.org>, David Vernet <void@...ifault.com>,
 Andrea Righi <arighi@...dia.com>, Changwoo Min <changwoo@...lia.com>,
 Shuah Khan <shuah@...nel.org>,
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 Alan Maguire <alan.maguire@...cle.com>, Donglin Peng <dolinux.peng@...il.com>
Cc: bpf@...r.kernel.org, dwarves@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 3/6] resolve_btfids: Introduce enum
 btf_id_kind

On 12/11/25 11:09 PM, Eduard Zingerman wrote:
> On Fri, 2025-12-05 at 14:30 -0800, Ihor Solodrai wrote:
>> Instead of using multiple flags, make struct btf_id tagged with an
>> enum value indicating its kind in the context of resolve_btfids.
>>
>> Signed-off-by: Ihor Solodrai <ihor.solodrai@...ux.dev>
>> ---
> 
> Acked-by: Eduard Zingerman <eddyz87@...il.com>
> 
> (But see a question below).
> 
>> @@ -213,14 +218,19 @@ btf_id__add(struct rb_root *root, char *name, bool unique)
>>  			p = &(*p)->rb_left;
>>  		else if (cmp > 0)
>>  			p = &(*p)->rb_right;
>> -		else
>> -			return unique ? NULL : id;
>> +		else if (kind == BTF_ID_KIND_SYM && id->kind == BTF_ID_KIND_SYM)
> 
> Nit: I'd keep the 'unique' parameter alongside 'kind' and resolve this
>      condition on the function callsite.

I don't like the boolean args, they're always opaque on the callsite.

We want to allow duplicates for _KIND_SYM and forbid for other kinds.
Since we are passing the kind from outside, I think it makes sense to
check for this inside the function. It makes the usage simpler.

> 
>> +			return id;
>> +		else {
>> +			pr_err("Unexpected duplicate symbol %s of kind %d\n", name, id->kind);
>> +			return NULL;
>> +		}
> 
> [...]
> 
>> @@ -491,28 +515,24 @@ static int symbols_collect(struct object *obj)
>>  			id = add_symbol(&obj->funcs, prefix, sizeof(BTF_FUNC) - 1);
>>  		/* set8 */
>>  		} else if (!strncmp(prefix, BTF_SET8, sizeof(BTF_SET8) - 1)) {
>> -			id = add_set(obj, prefix, true);
>> +			id = add_set(obj, prefix, BTF_ID_KIND_SET8);
>>  			/*
>>  			 * SET8 objects store list's count, which is encoded
>>  			 * in symbol's size, together with 'cnt' field hence
>>  			 * that - 1.
>>  			 */
>> -			if (id) {
>> +			if (id)
>>  				id->cnt = sym.st_size / sizeof(uint64_t) - 1;
>> -				id->is_set8 = true;
>> -			}
>>  		/* set */
>>  		} else if (!strncmp(prefix, BTF_SET, sizeof(BTF_SET) - 1)) {
>> -			id = add_set(obj, prefix, false);
>> +			id = add_set(obj, prefix, BTF_ID_KIND_SET);
>>  			/*
>>  			 * SET objects store list's count, which is encoded
>>  			 * in symbol's size, together with 'cnt' field hence
>>  			 * that - 1.
>>  			 */
>> -			if (id) {
>> +			if (id)
> 
> Current patch is not a culprit, but shouldn't resolve_btfids fail if
> `id` cannot be added? (here and in a hunk above).

By the existing design, resolve_btfids generally fails if
CONFIG_WERROR is set and `warnings > 0`.

And in this particular place it would fails with -ENOMEM a bit below:

       [...]
		} else if (!strncmp(prefix, BTF_SET, sizeof(BTF_SET) - 1)) {
			id = add_set(obj, prefix, BTF_ID_KIND_SET);
			/*
			 * SET objects store list's count, which is encoded
			 * in symbol's size, together with 'cnt' field hence
			 * that - 1.
			 */
			if (id)
				id->cnt = sym.st_size / sizeof(int) - 1;
		} else {
			pr_err("FAILED unsupported prefix %s\n", prefix);
			return -1;
		}

  /* --> */	if (!id)
			return -ENOMEM;

So I think an error code change may be appropriate, and that's about it.

> 
>>  				id->cnt = sym.st_size / sizeof(int) - 1;
>> -				id->is_set = true;
>> -			}
>>  		} else {
>>  			pr_err("FAILED unsupported prefix %s\n", prefix);
>>  			return -1;
> 
> [...]
>