Add special BPF CO-RE mode to `btf dump` sub-command, which emits C dump of
provided BTF data, but with additional provisions for use with BPF CO-RE.
The first special BPF CO-RE specific feature added is applying
preserve_access_index attribute to all structs/unions to make them
automatically relocatable. This is especially useful for tp_btf/fentry/fexit
BPF program types. They allow direct memory access, so BPF C code just uses
straightfoward a->b->c access pattern to read data from kernel. But without
kernel structs marked as CO-RE relocatable through preserve_access_index
attribute, one has to enclose all the data reads into a special
__builtin_preserve_access_index code block, like so:
__builtin_preserve_access_index(({
x = p->pid; /* where p is struct task_struct *, for example */
}));
This is very inconvenient and obscures the logic quite a bit. By marking all
auto-generated types with preserve_access_index attribute the above code is
reduced to just clean and natural `x = p->pid;`.
Having a special `format core`, as opposed to extending existing `format c`
mode, allows us to do more improvements like above, knowing that intended use
case is for BPF CO-RE. And still have a clean and unassuming plain C mode for
any other generic use case.
Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
---
.../bpf/bpftool/Documentation/bpftool-btf.rst | 7 ++++--
tools/bpf/bpftool/bash-completion/bpftool | 2 +-
tools/bpf/bpftool/btf.c | 24 +++++++++++++++----
3 files changed, 26 insertions(+), 7 deletions(-)
diff --git a/tools/bpf/bpftool/Documentation/bpftool-btf.rst b/tools/bpf/bpftool/Documentation/bpftool-btf.rst
index 39615f8e145b..101a91280f3d 100644
--- a/tools/bpf/bpftool/Documentation/bpftool-btf.rst
+++ b/tools/bpf/bpftool/Documentation/bpftool-btf.rst
@@ -24,7 +24,7 @@ BTF COMMANDS
| **bpftool** **btf help**
|
| *BTF_SRC* := { **id** *BTF_ID* | **prog** *PROG* | **map** *MAP* [{**key** | **value** | **kv** | **all**}] | **file** *FILE* }
-| *FORMAT* := { **raw** | **c** }
+| *FORMAT* := { **raw** | **c** | **core** }
| *MAP* := { **id** *MAP_ID* | **pinned** *FILE* }
| *PROG* := { **id** *PROG_ID* | **pinned** *FILE* | **tag** *PROG_TAG* }
@@ -59,7 +59,10 @@ DESCRIPTION
**format** option can be used to override default (raw)
output format. Raw (**raw**) or C-syntax (**c**) output
- formats are supported.
+ formats are supported. There is also a special BPF CO-RE
+ mode (**core**), which emits types in valid C syntax, but
+ with additional provisions for more seamless use with BPF
+ CO-RE.
**bpftool btf help**
Print short help message.
diff --git a/tools/bpf/bpftool/bash-completion/bpftool b/tools/bpf/bpftool/bash-completion/bpftool
index 754d8395e451..8308ae5f5679 100644
--- a/tools/bpf/bpftool/bash-completion/bpftool
+++ b/tools/bpf/bpftool/bash-completion/bpftool
@@ -800,7 +800,7 @@ _bpftool()
return 0
;;
format)
- COMPREPLY=( $( compgen -W "c raw" -- "$cur" ) )
+ COMPREPLY=( $( compgen -W "c core raw" -- "$cur" ) )
;;
*)
# emit extra options
diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index e5bc97b71ceb..a57494493fec 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -361,7 +361,7 @@ static void __printf(2, 0) btf_dump_printf(void *ctx,
}
static int dump_btf_c(const struct btf *btf,
- __u32 *root_type_ids, int root_type_cnt)
+ __u32 *root_type_ids, int root_type_cnt, bool core_mode)
{
struct btf_dump *d;
int err = 0, i;
@@ -370,6 +370,13 @@ static int dump_btf_c(const struct btf *btf,
if (IS_ERR(d))
return PTR_ERR(d);
+ if (core_mode) {
+ printf("#if defined(__has_attribute) && __has_attribute(preserve_access_index)\n");
+ printf("#define __CLANG_BPF_CORE_SUPPORTED\n");
+ printf("#pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)\n");
+ printf("#endif\n\n");
+ }
+
if (root_type_cnt) {
for (i = 0; i < root_type_cnt; i++) {
err = btf_dump__dump_type(d, root_type_ids[i]);
@@ -386,6 +393,12 @@ static int dump_btf_c(const struct btf *btf,
}
}
+ if (core_mode) {
+ printf("#ifdef __CLANG_BPF_CORE_SUPPORTED\n");
+ printf("#pragma clang attribute pop\n");
+ printf("#endif\n");
+ }
+
done:
btf_dump__free(d);
return err;
@@ -441,10 +454,10 @@ static bool is_btf_raw(const char *file)
static int do_dump(int argc, char **argv)
{
+ bool dump_c = false, core_mode = false;
struct btf *btf = NULL;
__u32 root_type_ids[2];
int root_type_cnt = 0;
- bool dump_c = false;
__u32 btf_id = -1;
const char *src;
int fd = -1;
@@ -544,6 +557,9 @@ static int do_dump(int argc, char **argv)
}
if (strcmp(*argv, "c") == 0) {
dump_c = true;
+ } else if (strcmp(*argv, "core") == 0) {
+ dump_c = true;
+ core_mode = true;
} else if (strcmp(*argv, "raw") == 0) {
dump_c = false;
} else {
@@ -577,7 +593,7 @@ static int do_dump(int argc, char **argv)
err = -ENOTSUP;
goto done;
}
- err = dump_btf_c(btf, root_type_ids, root_type_cnt);
+ err = dump_btf_c(btf, root_type_ids, root_type_cnt, core_mode);
} else {
err = dump_btf_raw(btf, root_type_ids, root_type_cnt);
}
@@ -925,7 +941,7 @@ static int do_help(int argc, char **argv)
" %s btf help\n"
"\n"
" BTF_SRC := { id BTF_ID | prog PROG | map MAP [{key | value | kv | all}] | file FILE }\n"
- " FORMAT := { raw | c }\n"
+ " FORMAT := { raw | c | core }\n"
" " HELP_SPEC_MAP "\n"
" " HELP_SPEC_PROGRAM "\n"
" " HELP_SPEC_OPTIONS "\n"
--
2.17.1
