On 12/5/20 12:35 PM, Yonghong Song wrote:
On 12/4/20 10:48 AM, Alan Maguire wrote:
This series aims to add support to bpf_snprintf_btf() and
bpf_seq_printf_btf() allowing them to store string representations
of module-specific types, as well as the kernel-specific ones
they currently support.
Patch 1 removes the btf_module_mutex, as since we will need to
look up module BTF during BPF program execution, we don't want
to risk sleeping in the various contexts in which BPF can run.
The access patterns to the btf module list seem to conform to
classic list RCU usage so with a few minor tweaks this seems
workable.
Patch 2 replaces the unused flags field in struct btf_ptr with
an obj_id field, allowing the specification of the id of a
BTF module. If the value is 0, the core kernel vmlinux is
assumed to contain the type's BTF information. Otherwise the
module with that id is used to identify the type. If the
object-id based lookup fails, we again fall back to vmlinux
BTF.
Patch 3 is a selftest that uses veth (when built as a
module) and a kprobe to display both a module-specific
and kernel-specific type; both are arguments to veth_stats_rx().
Currently it looks up the module-specific type and object ids
using libbpf; in future, these lookups will likely be supported
directly in the BPF program via __builtin_btf_type_id(); but
I need to determine a good test to determine if that builtin
supports object ids.
__builtin_btf_type_id() is really only supported in llvm12
and 64bit return value support is pushed to llvm12 trunk
a while back. The builtin is introduced in llvm11 but has a
corner bug, so llvm12 is recommended. So if people use the builtin,
you can assume 64bit return value. libbpf support is required
here. So in my opinion, there is no need to do feature detection.
if people use llvm11 which may cause test to fail, we can add
an entry in selftest README file to warn people this specific
test needs llvm12.
Andrii has a patch to support 64bit return value for
__builtin_btf_type_id() and I assume that one should
be landed before or together with your patch.
Just for your info. The following is an example you could
use to determine whether __builtin_btf_type_id()
supports btf object id at llvm level.
-bash-4.4$ cat t.c
int test(int arg) {
return __builtin_btf_type_id(arg, 1);
}
Compile to generate assembly code with latest llvm12 trunk:
clang -target bpf -O2 -S -g -mcpu=v3 t.c
In the asm code, you should see one line with
r0 = 1 ll
Or you can generate obj code:
clang -target bpf -O2 -c -g -mcpu=v3 t.c
and then you disassemble the obj file
llvm-objdump -d --no-show-raw-insn --no-leading-addr t.o
You should see below in the output
r0 = 1 ll
Use earlier version of llvm12 trunk, the builtin has
32bit return value, you will see
r0 = 1
which is a 32bit imm to r0, while "r0 = 1 ll" is
64bit imm to r0.
Changes since RFC
- add patch to remove module mutex
- modify to use obj_id instead of module name as identifier
in "struct btf_ptr" (Andrii)
Alan Maguire (3):
bpf: eliminate btf_module_mutex as RCU synchronization can be used
bpf: add module support to btf display helpers
selftests/bpf: verify module-specific types can be shown via
bpf_snprintf_btf
include/linux/btf.h | 12 ++
include/uapi/linux/bpf.h | 13 ++-
kernel/bpf/btf.c | 49 +++++---
kernel/trace/bpf_trace.c | 44 ++++++--
tools/include/uapi/linux/bpf.h | 13 ++-
.../selftests/bpf/prog_tests/snprintf_btf_mod.c | 124
+++++++++++++++++++++
tools/testing/selftests/bpf/progs/bpf_iter.h | 2 +-
tools/testing/selftests/bpf/progs/btf_ptr.h | 2 +-
tools/testing/selftests/bpf/progs/veth_stats_rx.c | 72 ++++++++++++
9 files changed, 292 insertions(+), 39 deletions(-)
create mode 100644
tools/testing/selftests/bpf/prog_tests/snprintf_btf_mod.c
create mode 100644 tools/testing/selftests/bpf/progs/veth_stats_rx.c
