Re: [PATCH v2 bpf-next 1/2] bpf: add a ksym BPF iterator

[Yonghong Song <yhs@fb.com>]

On 7/1/22 2:26 AM, Alan Maguire wrote:
> add a "ksym" iterator which provides access to a "struct kallsym_iter"
> for each symbol.  Intent is to support more flexible symbol parsing
> as discussed in [1].
> 
> [1] https://lore.kernel.org/all/YjRPZj6Z8vuLeEZo@krava/
> 
> Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> ---
>   kernel/kallsyms.c | 89 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 89 insertions(+)
> 
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index fbdf8d3..8b662da 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -30,6 +30,7 @@
>   #include <linux/module.h>
>   #include <linux/kernel.h>
>   #include <linux/bsearch.h>
> +#include <linux/btf_ids.h>
>   
>   /*
>    * These will be re-linked against their real values
> @@ -799,6 +800,91 @@ static int s_show(struct seq_file *m, void *p)
>   	.show = s_show
>   };
>   
> +#ifdef CONFIG_BPF_SYSCALL
> +
> +struct bpf_iter__ksym {
> +	__bpf_md_ptr(struct bpf_iter_meta *, meta);
> +	__bpf_md_ptr(struct kallsym_iter *, ksym);
> +};
> +
> +static int ksym_prog_seq_show(struct seq_file *m, bool in_stop)
> +{
> +	struct bpf_iter__ksym ctx;
> +	struct bpf_iter_meta meta;
> +	struct bpf_prog *prog;
> +
> +	meta.seq = m;
> +	prog = bpf_iter_get_info(&meta, in_stop);
> +	if (!prog)
> +		return 0;
> +
> +	ctx.meta = &meta;
> +	ctx.ksym = m ? m->private : NULL;
> +	return bpf_iter_run_prog(prog, &ctx);
> +}
> +
> +static int bpf_iter_ksym_seq_show(struct seq_file *m, void *p)
> +{
> +	return ksym_prog_seq_show(m, false);
> +}
> +
> +static void bpf_iter_ksym_seq_stop(struct seq_file *m, void *p)
> +{
> +	if (!p)
> +		(void) ksym_prog_seq_show(m, true);
> +	else
> +		s_stop(m, p);
> +}
> +
> +static const struct seq_operations bpf_iter_ksym_ops = {
> +	.start = s_start,
> +	.next = s_next,
> +	.stop = bpf_iter_ksym_seq_stop,
> +	.show = bpf_iter_ksym_seq_show,
> +};
> +
> +static int bpf_iter_ksym_init(void *priv_data, struct bpf_iter_aux_info *aux)
> +{
> +	struct kallsym_iter *iter = priv_data;
> +
> +	reset_iter(iter, 0);
> +
> +	iter->show_value = true;

I think instead of always having show_value = true, we should have
    iter->show_value = kallsyms_show_value(...);

this is consistent with what `cat /proc/kallsyms` is doing, and
also consistent with bpf_dump_raw_ok() used when dumping various
kernel info in syscall.c.

We don't have a file here, so credential can be from the current
process with current_cred().

> +
> +	return 0;
> +}
> +
> +DEFINE_BPF_ITER_FUNC(ksym, struct bpf_iter_meta *meta, struct kallsym_iter *ksym)
> +
> +static const struct bpf_iter_seq_info ksym_iter_seq_info = {
> +	.seq_ops		= &bpf_iter_ksym_ops,
> +	.init_seq_private	= bpf_iter_ksym_init,
> +	.fini_seq_private	= NULL,
> +	.seq_priv_size		= sizeof(struct kallsym_iter),
> +};
> +
> +static struct bpf_iter_reg ksym_iter_reg_info = {
> +	.target                 = "ksym",
> +	.ctx_arg_info_size	= 1,
> +	.ctx_arg_info		= {
> +		{ offsetof(struct bpf_iter__ksym, ksym),
> +		  PTR_TO_BTF_ID_OR_NULL },
> +	},
> +	.seq_info		= &ksym_iter_seq_info,
> +};
> +
> +BTF_ID_LIST(btf_ksym_iter_id)
> +BTF_ID(struct, kallsym_iter)
> +
> +static void __init bpf_ksym_iter_register(void)
> +{
> +	ksym_iter_reg_info.ctx_arg_info[0].btf_id = *btf_ksym_iter_id;
> +	if (bpf_iter_reg_target(&ksym_iter_reg_info))
> +		pr_warn("Warning: could not register bpf ksym iterator\n");
> +}
> +
> +#endif /* CONFIG_BPF_SYSCALL */
> +
>   static inline int kallsyms_for_perf(void)
>   {
>   #ifdef CONFIG_PERF_EVENTS
> @@ -885,6 +971,9 @@ const char *kdb_walk_kallsyms(loff_t *pos)
>   static int __init kallsyms_init(void)
>   {
>   	proc_create("kallsyms", 0444, NULL, &kallsyms_proc_ops);
> +#if defined(CONFIG_BPF_SYSCALL)
> +	bpf_ksym_iter_register();

You can inline this function here and if bpf_iter_reg_target(...) 
failed, just return the error code.

> +#endif
>   	return 0;
>   }
>   device_initcall(kallsyms_init);