On Tue, Apr 27, 2021 at 09:34:30AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Tue, Apr 27, 2021 at 01:38:20PM +0200, Jiri Olsa escreveu:
> > On Mon, Apr 26, 2021 at 04:26:11PM -0700, Andrii Nakryiko wrote:
> > > On Fri, Apr 23, 2021 at 2:37 PM Martin KaFai Lau <kafai@xxxxxx> wrote:
> > > > BTF is currently generated for functions that are in ftrace list
> > > > or extern.
> > > >
> > > > A recent use case also needs BTF generated for functions included in
> > > > allowlist. In particular, the kernel
> > > > commit e78aea8b2170 ("bpf: tcp: Put some tcp cong functions in allowlist for bpf-tcp-cc")
> > > > allows bpf program to directly call a few tcp cc kernel functions. Those
> > > > functions are specified under an ELF section .BTF_ids. The symbols
> > > > in this ELF section is like __BTF_ID__func__<kernel_func>__[digit]+.
> > > > For example, __BTF_ID__func__cubictcp_init__1. Those kernel
> > > > functions are currently allowed only if CONFIG_DYNAMIC_FTRACE is
> > > > set to ensure they are in the ftrace list but this kconfig dependency
> > > > is unnecessary.
> > > >
> > > > pahole can generate BTF for those kernel functions if it knows they
> > > > are in the allowlist. This patch is to capture those symbols
> > > > in the .BTF_ids section and generate BTF for them.
>
> > > I wonder if we just record all functions how bad that would be. Jiri,
> > > do you remember from the time you were experimenting with static
> > > functions how much more functions we'd be recording if we didn't do
> > > ftrace filtering?
>
> > hum, I can't find that.. but should be just matter of removing
> > that is_ftrace_func check
>
> > if we decided to do that, maybe we could add some bit indicating
> > that the function is traceable? it would save us check with
> > available_filter_functions file
>
> You mean encoding it in BTF, in 'struct btf_type'? Seems important to
> have it, there are free bits there:
>
> /* Max # of type identifier */
> #define BTF_MAX_TYPE 0x000fffff
> /* Max offset into the string section */
> #define BTF_MAX_NAME_OFFSET 0x00ffffff
> /* Max # of struct/union/enum members or func args */
> #define BTF_MAX_VLEN 0xffff
>
> struct btf_type {
> __u32 name_off;
> /* "info" bits arrangement
> * bits 0-15: vlen (e.g. # of struct's members)
> * bits 16-23: unused
> * bits 24-27: kind (e.g. int, ptr, array...etc)
> * bits 28-30: unused
> * bit 31: kind_flag, currently used by
> * struct, union and fwd
> */
> __u32 info;
> /* "size" is used by INT, ENUM, STRUCT, UNION and DATASEC.
> * "size" tells the size of the type it is describing.
> *
> * "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT,
> * FUNC, FUNC_PROTO and VAR.
> * "type" is a type_id referring to another type.
> */
> union {
> __u32 size;
> __u32 type;
> };
> };
>
> And tools that expect to trace a function can get that information from
> the BTF info instead of getting some failure when trying to trace those
> functions, right?
yep, if it's possible to spare some bit for this info
jirka
