In order to enable the older kernel to use the CO-RE feature, load the
vmlinux btf of the specified path.
Learn from Andrii's comments in [0], add the custom_btf_path parameter
to bpf_obj_open_opts, you can directly use the skeleton's
<objname>_bpf__open_opts function to pass in the custom_btf_path
parameter.
Prior to this, there was also a developer who provided a patch with
similar functions. It is a pity that the follow-up did not continue to
advance. See [1].
[0]https://lore.kernel.org/bpf/CAEf4BzbJZLjNoiK8_VfeVg_Vrg=9iYFv+po-38SMe=UzwDKJ=Q@mail.gmail.com/#t
[1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/
Signed-off-by: Shuyi Cheng <[email protected]>
---
tools/lib/bpf/libbpf.c | 23 ++++++++++++++++++++---
tools/lib/bpf/libbpf.h | 6 +++++-
2 files changed, 25 insertions(+), 4 deletions(-)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 1e04ce7..518b19f 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -509,6 +509,8 @@ struct bpf_object {
void *priv;
bpf_object_clear_priv_t clear_priv;
+ char *custom_btf_path;
+
char path[];
};
#define obj_elf_valid(o) ((o)->efile.elf)
@@ -2679,8 +2681,15 @@ static int bpf_object__load_vmlinux_btf(struct bpf_object *obj, bool force)
if (!force && !obj_needs_vmlinux_btf(obj))
return 0;
- obj->btf_vmlinux = libbpf_find_kernel_btf();
- err = libbpf_get_error(obj->btf_vmlinux);
+ if (obj->custom_btf_path) {
+ obj->btf_vmlinux = btf__parse(obj->custom_btf_path, NULL);
+ err = libbpf_get_error(obj->btf_vmlinux);
+ pr_debug("loading custom vmlinux BTF '%s': %d\n", obj->custom_btf_path, err);
+ } else {
+ obj->btf_vmlinux = libbpf_find_kernel_btf();
+ err = libbpf_get_error(obj->btf_vmlinux);
+ }
+
if (err) {
pr_warn("Error loading vmlinux BTF: %d\n", err);
obj->btf_vmlinux = NULL;
@@ -7554,7 +7563,7 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
__bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
const struct bpf_object_open_opts *opts)
{
- const char *obj_name, *kconfig;
+ const char *obj_name, *kconfig, *tmp_btf_path;
struct bpf_program *prog;
struct bpf_object *obj;
char tmp_name[64];
@@ -7584,6 +7593,13 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
obj = bpf_object__new(path, obj_buf, obj_buf_sz, obj_name);
if (IS_ERR(obj))
return obj;
+
+ tmp_btf_path = OPTS_GET(opts, custom_btf_path, NULL);
+ if (tmp_btf_path && strlen(tmp_btf_path) < PATH_MAX) {
+ obj->custom_btf_path = strdup(tmp_btf_path);
+ if (!obj->custom_btf_path)
+ return ERR_PTR(-ENOMEM);
+ }
kconfig = OPTS_GET(opts, kconfig, NULL);
if (kconfig) {
@@ -8702,6 +8718,7 @@ void bpf_object__close(struct bpf_object *obj)
for (i = 0; i < obj->nr_maps; i++)
bpf_map__destroy(&obj->maps[i]);
+ zfree(&obj->custom_btf_path);
zfree(&obj->kconfig);
zfree(&obj->externs);
obj->nr_extern = 0;
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 6e61342..16e0f01 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -94,8 +94,12 @@ struct bpf_object_open_opts {
* system Kconfig for CONFIG_xxx externs.
*/
const char *kconfig;
+ /* Specify the path of vmlinux btf to facilitate the use of CO-RE features
+ * in the old kernel.
+ */
+ char *custom_btf_path;
};
-#define bpf_object_open_opts__last_field kconfig
+#define bpf_object_open_opts__last_field custom_btf_path
LIBBPF_API struct bpf_object *bpf_object__open(const char *path);
LIBBPF_API struct bpf_object *
--
1.8.3.1
On 6/24/21 6:03 AM, Shuyi Cheng wrote:
> In order to enable the older kernel to use the CO-RE feature, load the
> vmlinux btf of the specified path.
>
> Learn from Andrii's comments in [0], add the custom_btf_path parameter
> to bpf_obj_open_opts, you can directly use the skeleton's
> <objname>_bpf__open_opts function to pass in the custom_btf_path
> parameter.
>
> Prior to this, there was also a developer who provided a patch with
> similar functions. It is a pity that the follow-up did not continue to
> advance. See [1].
>
> [0]https://lore.kernel.org/bpf/CAEf4BzbJZLjNoiK8_VfeVg_Vrg=9iYFv+po-38SMe=UzwDKJ=Q@mail.gmail.com/#t
> [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/
>
> Signed-off-by: Shuyi Cheng <[email protected]>
> ---
> tools/lib/bpf/libbpf.c | 23 ++++++++++++++++++++---
> tools/lib/bpf/libbpf.h | 6 +++++-
> 2 files changed, 25 insertions(+), 4 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 1e04ce7..518b19f 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -509,6 +509,8 @@ struct bpf_object {
> void *priv;
> bpf_object_clear_priv_t clear_priv;
>
> + char *custom_btf_path;
> +
nit: This should rather go to the 'Parse and load BTF vmlinux if any of [...]'
section of struct bpf_object, and for consistency, I'd keep the btf_ prefix,
like: char *btf_custom_path
> char path[];
> };
> #define obj_elf_valid(o) ((o)->efile.elf)
> @@ -2679,8 +2681,15 @@ static int bpf_object__load_vmlinux_btf(struct bpf_object *obj, bool force)
> if (!force && !obj_needs_vmlinux_btf(obj))
> return 0;
>
> - obj->btf_vmlinux = libbpf_find_kernel_btf();
> - err = libbpf_get_error(obj->btf_vmlinux);
> + if (obj->custom_btf_path) {
> + obj->btf_vmlinux = btf__parse(obj->custom_btf_path, NULL);
> + err = libbpf_get_error(obj->btf_vmlinux);
> + pr_debug("loading custom vmlinux BTF '%s': %d\n", obj->custom_btf_path, err);
> + } else {
> + obj->btf_vmlinux = libbpf_find_kernel_btf();
> + err = libbpf_get_error(obj->btf_vmlinux);
> + }
Couldn't we do something like (only compile-tested):
diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index b46760b93bb4..5b88ce3e483c 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -4394,7 +4394,7 @@ static int btf_dedup_remap_types(struct btf_dedup *d)
* Probe few well-known locations for vmlinux kernel image and try to load BTF
* data out of it to use for target BTF.
*/
-struct btf *libbpf_find_kernel_btf(void)
+static struct btf *__libbpf_find_kernel_btf(char *btf_custom_path)
{
struct {
const char *path_fmt;
@@ -4402,6 +4402,8 @@ struct btf *libbpf_find_kernel_btf(void)
} locations[] = {
/* try canonical vmlinux BTF through sysfs first */
{ "/sys/kernel/btf/vmlinux", true /* raw BTF */ },
+ /* try user defined vmlinux ELF if a path was specified */
+ { btf_custom_path },
/* fall back to trying to find vmlinux ELF on disk otherwise */
{ "/boot/vmlinux-%1$s" },
{ "/lib/modules/%1$s/vmlinux-%1$s" },
@@ -4419,11 +4421,11 @@ struct btf *libbpf_find_kernel_btf(void)
uname(&buf);
for (i = 0; i < ARRAY_SIZE(locations); i++) {
+ if (!locations[i].path_fmt)
+ continue;
snprintf(path, PATH_MAX, locations[i].path_fmt, buf.release);
-
if (access(path, R_OK))
continue;
-
if (locations[i].raw_btf)
btf = btf__parse_raw(path);
else
@@ -4440,6 +4442,11 @@ struct btf *libbpf_find_kernel_btf(void)
return libbpf_err_ptr(-ESRCH);
}
+struct btf *libbpf_find_kernel_btf(void)
+{
+ return __libbpf_find_kernel_btf(NULL);
+}
+
int btf_type_visit_type_ids(struct btf_type *t, type_id_visit_fn visit, void *ctx)
{
int i, n, err;
And then you just call it as:
obj->btf_vmlinux = __libbpf_find_kernel_btf(obj->btf_custom_path);
err = libbpf_get_error(obj->btf_vmlinux);
> if (err) {
> pr_warn("Error loading vmlinux BTF: %d\n", err);
> obj->btf_vmlinux = NULL;
> @@ -7554,7 +7563,7 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
> __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
> const struct bpf_object_open_opts *opts)
> {
> - const char *obj_name, *kconfig;
> + const char *obj_name, *kconfig, *tmp_btf_path;
> struct bpf_program *prog;
> struct bpf_object *obj;
> char tmp_name[64];
> @@ -7584,6 +7593,13 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
> obj = bpf_object__new(path, obj_buf, obj_buf_sz, obj_name);
> if (IS_ERR(obj))
> return obj;
> +
> + tmp_btf_path = OPTS_GET(opts, custom_btf_path, NULL);
> + if (tmp_btf_path && strlen(tmp_btf_path) < PATH_MAX) {
> + obj->custom_btf_path = strdup(tmp_btf_path);
> + if (!obj->custom_btf_path)
> + return ERR_PTR(-ENOMEM);
> + }
>
> kconfig = OPTS_GET(opts, kconfig, NULL);
> if (kconfig) {
> @@ -8702,6 +8718,7 @@ void bpf_object__close(struct bpf_object *obj)
> for (i = 0; i < obj->nr_maps; i++)
> bpf_map__destroy(&obj->maps[i]);
>
> + zfree(&obj->custom_btf_path);
> zfree(&obj->kconfig);
> zfree(&obj->externs);
> obj->nr_extern = 0;
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 6e61342..16e0f01 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -94,8 +94,12 @@ struct bpf_object_open_opts {
> * system Kconfig for CONFIG_xxx externs.
> */
> const char *kconfig;
> + /* Specify the path of vmlinux btf to facilitate the use of CO-RE features
> + * in the old kernel.
> + */
> + char *custom_btf_path;
> };
> -#define bpf_object_open_opts__last_field kconfig
> +#define bpf_object_open_opts__last_field custom_btf_path
>
> LIBBPF_API struct bpf_object *bpf_object__open(const char *path);
> LIBBPF_API struct bpf_object *
>
On 6/24/21 11:06 PM, Daniel Borkmann wrote:
> On 6/24/21 6:03 AM, Shuyi Cheng wrote:
>> In order to enable the older kernel to use the CO-RE feature, load the
>> vmlinux btf of the specified path.
>>
>> Learn from Andrii's comments in [0], add the custom_btf_path parameter
>> to bpf_obj_open_opts, you can directly use the skeleton's
>> <objname>_bpf__open_opts function to pass in the custom_btf_path
>> parameter.
>>
>> Prior to this, there was also a developer who provided a patch with
>> similar functions. It is a pity that the follow-up did not continue to
>> advance. See [1].
>>
>> [0]https://lore.kernel.org/bpf/CAEf4BzbJZLjNoiK8_VfeVg_Vrg=9iYFv+po-38SMe=UzwDKJ=Q@mail.gmail.com/#t
>>
>> [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/
>>
>>
>> Signed-off-by: Shuyi Cheng <[email protected]>
>> ---
>> tools/lib/bpf/libbpf.c | 23 ++++++++++++++++++++---
>> tools/lib/bpf/libbpf.h | 6 +++++-
>> 2 files changed, 25 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 1e04ce7..518b19f 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -509,6 +509,8 @@ struct bpf_object {
>> void *priv;
>> bpf_object_clear_priv_t clear_priv;
>> + char *custom_btf_path;
>> +
>
> nit: This should rather go to the 'Parse and load BTF vmlinux if any of
> [...]'
> section of struct bpf_object, and for consistency, I'd keep the btf_
> prefix,
> like: char *btf_custom_path
>
Thank you very much for your reply.
Agree.
>> char path[];
>> };
>> #define obj_elf_valid(o) ((o)->efile.elf)
>> @@ -2679,8 +2681,15 @@ static int bpf_object__load_vmlinux_btf(struct
>> bpf_object *obj, bool force)
>> if (!force && !obj_needs_vmlinux_btf(obj))
>> return 0;
>> - obj->btf_vmlinux = libbpf_find_kernel_btf();
>> - err = libbpf_get_error(obj->btf_vmlinux);
>> + if (obj->custom_btf_path) {
>> + obj->btf_vmlinux = btf__parse(obj->custom_btf_path, NULL);
>> + err = libbpf_get_error(obj->btf_vmlinux);
>> + pr_debug("loading custom vmlinux BTF '%s': %d\n",
>> obj->custom_btf_path, err);
>> + } else {
>> + obj->btf_vmlinux = libbpf_find_kernel_btf();
>> + err = libbpf_get_error(obj->btf_vmlinux);
>> + }
>
> Couldn't we do something like (only compile-tested):
>
Your approach is very inspiring to me. But I did it for two reasons.
1. When the developer specifies btf_custom_path, btf should only be
loaded from btf_custom_path;
2. Now pahole supports saving vmlinux's btf in raw format, so the old
kernel can provide btf in elf format or raw format. see [0].
[0]
https://git.kernel.org/pub/scm/devel/pahole/pahole.git/tree/pahole.c#n1157
What do you think?
Regards,
Shuyi
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index b46760b93bb4..5b88ce3e483c 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -4394,7 +4394,7 @@ static int btf_dedup_remap_types(struct btf_dedup *d)
> * Probe few well-known locations for vmlinux kernel image and try to
> load BTF
> * data out of it to use for target BTF.
> */
> -struct btf *libbpf_find_kernel_btf(void)
> +static struct btf *__libbpf_find_kernel_btf(char *btf_custom_path)
> {
> struct {
> const char *path_fmt;
> @@ -4402,6 +4402,8 @@ struct btf *libbpf_find_kernel_btf(void)
> } locations[] = {
> /* try canonical vmlinux BTF through sysfs first */
> { "/sys/kernel/btf/vmlinux", true /* raw BTF */ },
> + /* try user defined vmlinux ELF if a path was specified */
> + { btf_custom_path },
> /* fall back to trying to find vmlinux ELF on disk otherwise */
> { "/boot/vmlinux-%1$s" },
> { "/lib/modules/%1$s/vmlinux-%1$s" },
> @@ -4419,11 +4421,11 @@ struct btf *libbpf_find_kernel_btf(void)
> uname(&buf);
>
> for (i = 0; i < ARRAY_SIZE(locations); i++) {
> + if (!locations[i].path_fmt)
> + continue;
> snprintf(path, PATH_MAX, locations[i].path_fmt, buf.release);
> -
> if (access(path, R_OK))
> continue;
> -
> if (locations[i].raw_btf)
> btf = btf__parse_raw(path);
> else
> @@ -4440,6 +4442,11 @@ struct btf *libbpf_find_kernel_btf(void)
> return libbpf_err_ptr(-ESRCH);
> }
>
> +struct btf *libbpf_find_kernel_btf(void)
> +{
> + return __libbpf_find_kernel_btf(NULL);
> +}
> +
> int btf_type_visit_type_ids(struct btf_type *t, type_id_visit_fn
> visit, void *ctx)
> {
> int i, n, err;
>
> And then you just call it as:
>
> obj->btf_vmlinux = __libbpf_find_kernel_btf(obj->btf_custom_path);
> err = libbpf_get_error(obj->btf_vmlinux);
>
>> if (err) {
>> pr_warn("Error loading vmlinux BTF: %d\n", err);
>> obj->btf_vmlinux = NULL;
On Thu, Jun 24, 2021 at 8:06 AM Daniel Borkmann <[email protected]> wrote:
>
> On 6/24/21 6:03 AM, Shuyi Cheng wrote:
> > In order to enable the older kernel to use the CO-RE feature, load the
> > vmlinux btf of the specified path.
> >
> > Learn from Andrii's comments in [0], add the custom_btf_path parameter
> > to bpf_obj_open_opts, you can directly use the skeleton's
> > <objname>_bpf__open_opts function to pass in the custom_btf_path
> > parameter.
> >
> > Prior to this, there was also a developer who provided a patch with
> > similar functions. It is a pity that the follow-up did not continue to
> > advance. See [1].
> >
> > [0]https://lore.kernel.org/bpf/CAEf4BzbJZLjNoiK8_VfeVg_Vrg=9iYFv+po-38SMe=UzwDKJ=Q@mail.gmail.com/#t
> > [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/
> >
> > Signed-off-by: Shuyi Cheng <[email protected]>
> > ---
> > tools/lib/bpf/libbpf.c | 23 ++++++++++++++++++++---
> > tools/lib/bpf/libbpf.h | 6 +++++-
> > 2 files changed, 25 insertions(+), 4 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 1e04ce7..518b19f 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -509,6 +509,8 @@ struct bpf_object {
> > void *priv;
> > bpf_object_clear_priv_t clear_priv;
> >
> > + char *custom_btf_path;
> > +
>
> nit: This should rather go to the 'Parse and load BTF vmlinux if any of [...]'
> section of struct bpf_object, and for consistency, I'd keep the btf_ prefix,
> like: char *btf_custom_path
>
> > char path[];
> > };
> > #define obj_elf_valid(o) ((o)->efile.elf)
> > @@ -2679,8 +2681,15 @@ static int bpf_object__load_vmlinux_btf(struct bpf_object *obj, bool force)
> > if (!force && !obj_needs_vmlinux_btf(obj))
> > return 0;
> >
> > - obj->btf_vmlinux = libbpf_find_kernel_btf();
> > - err = libbpf_get_error(obj->btf_vmlinux);
> > + if (obj->custom_btf_path) {
> > + obj->btf_vmlinux = btf__parse(obj->custom_btf_path, NULL);
> > + err = libbpf_get_error(obj->btf_vmlinux);
> > + pr_debug("loading custom vmlinux BTF '%s': %d\n", obj->custom_btf_path, err);
> > + } else {
> > + obj->btf_vmlinux = libbpf_find_kernel_btf();
> > + err = libbpf_get_error(obj->btf_vmlinux);
> > + }
>
> Couldn't we do something like (only compile-tested):
I wonder what are the benefits of this approach, though. My
expectation is that if the user specifies a custom BTF path and BTF is
missing then the whole bpf_object load process should fail, but in
this case it will be silently ignored. Also, if custom BTF is
specified, that custom BTF has to be used even if
/sys/kernel/btf/vmlinux is present, but the patch below will still
prefer /sys/kernel/btf/vmlinux.
So the semantics is different. I'm not saying it's wrong, but I think
it means we need to discuss what behavior we are after first.
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index b46760b93bb4..5b88ce3e483c 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -4394,7 +4394,7 @@ static int btf_dedup_remap_types(struct btf_dedup *d)
> * Probe few well-known locations for vmlinux kernel image and try to load BTF
> * data out of it to use for target BTF.
> */
> -struct btf *libbpf_find_kernel_btf(void)
> +static struct btf *__libbpf_find_kernel_btf(char *btf_custom_path)
> {
> struct {
> const char *path_fmt;
[...]
On Wed, Jun 23, 2021 at 9:04 PM Shuyi Cheng
<[email protected]> wrote:
>
> In order to enable the older kernel to use the CO-RE feature, load the
> vmlinux btf of the specified path.
>
> Learn from Andrii's comments in [0], add the custom_btf_path parameter
> to bpf_obj_open_opts, you can directly use the skeleton's
> <objname>_bpf__open_opts function to pass in the custom_btf_path
> parameter.
>
> Prior to this, there was also a developer who provided a patch with
> similar functions. It is a pity that the follow-up did not continue to
> advance. See [1].
>
> [0]https://lore.kernel.org/bpf/CAEf4BzbJZLjNoiK8_VfeVg_Vrg=9iYFv+po-38SMe=UzwDKJ=Q@mail.gmail.com/#t
> [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/
>
> Signed-off-by: Shuyi Cheng <[email protected]>
> ---
> tools/lib/bpf/libbpf.c | 23 ++++++++++++++++++++---
> tools/lib/bpf/libbpf.h | 6 +++++-
> 2 files changed, 25 insertions(+), 4 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 1e04ce7..518b19f 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -509,6 +509,8 @@ struct bpf_object {
> void *priv;
> bpf_object_clear_priv_t clear_priv;
>
> + char *custom_btf_path;
> +
> char path[];
> };
> #define obj_elf_valid(o) ((o)->efile.elf)
> @@ -2679,8 +2681,15 @@ static int bpf_object__load_vmlinux_btf(struct bpf_object *obj, bool force)
> if (!force && !obj_needs_vmlinux_btf(obj))
> return 0;
>
> - obj->btf_vmlinux = libbpf_find_kernel_btf();
> - err = libbpf_get_error(obj->btf_vmlinux);
> + if (obj->custom_btf_path) {
> + obj->btf_vmlinux = btf__parse(obj->custom_btf_path, NULL);
> + err = libbpf_get_error(obj->btf_vmlinux);
> + pr_debug("loading custom vmlinux BTF '%s': %d\n", obj->custom_btf_path, err);
> + } else {
> + obj->btf_vmlinux = libbpf_find_kernel_btf();
> + err = libbpf_get_error(obj->btf_vmlinux);
> + }
I think it will be more flexible to treat custom_btf as an vmlinux BTF
override, just like [1] did. I can see how in some situations users
might want to treat this custom BTF as either a replacement of vmlinux
BTF or as an augmentation of vmlinux BTF for the purpose of extra
custom CO-RE relocations (e.g., something along the XDP hints that
were discussed recently). For now it's probably enough to implement
"custom BTF is a replacement for vmlinux BTF" policy and, if
necessary, add "custom BTF is in addition to vmlinux BTF" later with
extra opts flag/field.
Keep in mind that this custom BTF is only useful for BPF CO-RE
relocation. Any other kernel feature relying on vmlinux BTF (e.g.,
fentry) won't work with custom BTF because it expects correct BTF type
IDs.
> +
> if (err) {
> pr_warn("Error loading vmlinux BTF: %d\n", err);
> obj->btf_vmlinux = NULL;
> @@ -7554,7 +7563,7 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
> __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
> const struct bpf_object_open_opts *opts)
> {
> - const char *obj_name, *kconfig;
> + const char *obj_name, *kconfig, *tmp_btf_path;
> struct bpf_program *prog;
> struct bpf_object *obj;
> char tmp_name[64];
> @@ -7584,6 +7593,13 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
> obj = bpf_object__new(path, obj_buf, obj_buf_sz, obj_name);
> if (IS_ERR(obj))
> return obj;
> +
> + tmp_btf_path = OPTS_GET(opts, custom_btf_path, NULL);
> + if (tmp_btf_path && strlen(tmp_btf_path) < PATH_MAX) {
if strlen() is >= PATH_MAX you'll just silently ignore it? We should
either truncate silently (because PATH_MAX is totally reasonable
assumption) or error out.
> + obj->custom_btf_path = strdup(tmp_btf_path);
> + if (!obj->custom_btf_path)
> + return ERR_PTR(-ENOMEM);
> + }
>
> kconfig = OPTS_GET(opts, kconfig, NULL);
> if (kconfig) {
> @@ -8702,6 +8718,7 @@ void bpf_object__close(struct bpf_object *obj)
> for (i = 0; i < obj->nr_maps; i++)
> bpf_map__destroy(&obj->maps[i]);
>
> + zfree(&obj->custom_btf_path);
> zfree(&obj->kconfig);
> zfree(&obj->externs);
> obj->nr_extern = 0;
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 6e61342..16e0f01 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -94,8 +94,12 @@ struct bpf_object_open_opts {
> * system Kconfig for CONFIG_xxx externs.
> */
> const char *kconfig;
> + /* Specify the path of vmlinux btf to facilitate the use of CO-RE features
> + * in the old kernel.
> + */
> + char *custom_btf_path;
> };
> -#define bpf_object_open_opts__last_field kconfig
> +#define bpf_object_open_opts__last_field custom_btf_path
>
> LIBBPF_API struct bpf_object *bpf_object__open(const char *path);
> LIBBPF_API struct bpf_object *
> --
> 1.8.3.1
>
On 7/8/21 4:52 AM, Andrii Nakryiko wrote:
> On Wed, Jun 23, 2021 at 9:04 PM Shuyi Cheng
> <[email protected]> wrote:
>>
>> In order to enable the older kernel to use the CO-RE feature, load the
>> vmlinux btf of the specified path.
>>
>> Learn from Andrii's comments in [0], add the custom_btf_path parameter
>> to bpf_obj_open_opts, you can directly use the skeleton's
>> <objname>_bpf__open_opts function to pass in the custom_btf_path
>> parameter.
>>
>> Prior to this, there was also a developer who provided a patch with
>> similar functions. It is a pity that the follow-up did not continue to
>> advance. See [1].
>>
>> [0]https://lore.kernel.org/bpf/CAEf4BzbJZLjNoiK8_VfeVg_Vrg=9iYFv+po-38SMe=UzwDKJ=Q@mail.gmail.com/#t
>> [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/
>>
>> Signed-off-by: Shuyi Cheng <[email protected]>
>> ---
>> tools/lib/bpf/libbpf.c | 23 ++++++++++++++++++++---
>> tools/lib/bpf/libbpf.h | 6 +++++-
>> 2 files changed, 25 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 1e04ce7..518b19f 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -509,6 +509,8 @@ struct bpf_object {
>> void *priv;
>> bpf_object_clear_priv_t clear_priv;
>>
>> + char *custom_btf_path;
>> +
>> char path[];
>> };
>> #define obj_elf_valid(o) ((o)->efile.elf)
>> @@ -2679,8 +2681,15 @@ static int bpf_object__load_vmlinux_btf(struct bpf_object *obj, bool force)
>> if (!force && !obj_needs_vmlinux_btf(obj))
>> return 0;
>>
>> - obj->btf_vmlinux = libbpf_find_kernel_btf();
>> - err = libbpf_get_error(obj->btf_vmlinux);
>> + if (obj->custom_btf_path) {
>> + obj->btf_vmlinux = btf__parse(obj->custom_btf_path, NULL);
>> + err = libbpf_get_error(obj->btf_vmlinux);
>> + pr_debug("loading custom vmlinux BTF '%s': %d\n", obj->custom_btf_path, err);
>> + } else {
>> + obj->btf_vmlinux = libbpf_find_kernel_btf();
>> + err = libbpf_get_error(obj->btf_vmlinux);
>> + }
>
>
> I think it will be more flexible to treat custom_btf as an vmlinux BTF
> override, just like [1] did. I can see how in some situations users
> might want to treat this custom BTF as either a replacement of vmlinux
> BTF or as an augmentation of vmlinux BTF for the purpose of extra
> custom CO-RE relocations (e.g., something along the XDP hints that
> were discussed recently). For now it's probably enough to implement
> "custom BTF is a replacement for vmlinux BTF" policy and, if
> necessary, add "custom BTF is in addition to vmlinux BTF" later with
> extra opts flag/field.
>
Thank you very much for your reply.
So this part of the code remains the same, right?
> Keep in mind that this custom BTF is only useful for BPF CO-RE
> relocation. Any other kernel feature relying on vmlinux BTF (e.g.,
> fentry) won't work with custom BTF because it expects correct BTF type
> IDs.
>
>
>> +
>> if (err) {
>> pr_warn("Error loading vmlinux BTF: %d\n", err);
>> obj->btf_vmlinux = NULL;
>> @@ -7554,7 +7563,7 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
>> __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
>> const struct bpf_object_open_opts *opts)
>> {
>> - const char *obj_name, *kconfig;
>> + const char *obj_name, *kconfig, *tmp_btf_path;
>> struct bpf_program *prog;
>> struct bpf_object *obj;
>> char tmp_name[64];
>> @@ -7584,6 +7593,13 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
>> obj = bpf_object__new(path, obj_buf, obj_buf_sz, obj_name);
>> if (IS_ERR(obj))
>> return obj;
>> +
>> + tmp_btf_path = OPTS_GET(opts, custom_btf_path, NULL);
>> + if (tmp_btf_path && strlen(tmp_btf_path) < PATH_MAX) {
>
> if strlen() is >= PATH_MAX you'll just silently ignore it? We should
> either truncate silently (because PATH_MAX is totally reasonable
> assumption) or error out.
Agree. I will send the second version of the patch.
Regards,
Shuyi
>
>> + obj->custom_btf_path = strdup(tmp_btf_path);
>> + if (!obj->custom_btf_path)
>> + return ERR_PTR(-ENOMEM);
>> + }
>>
>> kconfig = OPTS_GET(opts, kconfig, NULL);
>> if (kconfig) {
>> @@ -8702,6 +8718,7 @@ void bpf_object__close(struct bpf_object *obj)
>> for (i = 0; i < obj->nr_maps; i++)
>> bpf_map__destroy(&obj->maps[i]);
>>
>> + zfree(&obj->custom_btf_path);
>> zfree(&obj->kconfig);
>> zfree(&obj->externs);
>> obj->nr_extern = 0;
>> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>> index 6e61342..16e0f01 100644
>> --- a/tools/lib/bpf/libbpf.h
>> +++ b/tools/lib/bpf/libbpf.h
>> @@ -94,8 +94,12 @@ struct bpf_object_open_opts {
>> * system Kconfig for CONFIG_xxx externs.
>> */
>> const char *kconfig;
>> + /* Specify the path of vmlinux btf to facilitate the use of CO-RE features
>> + * in the old kernel.
>> + */
>> + char *custom_btf_path;
>> };
>> -#define bpf_object_open_opts__last_field kconfig
>> +#define bpf_object_open_opts__last_field custom_btf_path
>>
>> LIBBPF_API struct bpf_object *bpf_object__open(const char *path);
>> LIBBPF_API struct bpf_object *
>> --
>> 1.8.3.1
>>