On Wed, Jul 13, 2022 at 9:24 AM <sdf@xxxxxxxxxx> wrote:
>
> On 07/12, Andrii Nakryiko wrote:
> > Libbpf supports single virtual __kconfig extern currently:
> > LINUX_KERNEL_VERSION.
> > LINUX_KERNEL_VERSION isn't coming from /proc/kconfig.gz and is intead
> > customly filled out by libbpf.
>
> > This patch generalizes this approach to support more such virtual
> > __kconfig externs. One such extern added in this patch is
> > LINUX_HAS_BPF_COOKIE which is used for BPF-side USDT supporting code in
> > usdt.bpf.h instead of using CO-RE-based enum detection approach for
> > detecting bpf_get_attach_cookie() BPF helper. This allows to remove
> > otherwise not needed CO-RE dependency and keeps user-space and BPF-side
> > parts of libbpf's USDT support strictly in sync in terms of their
> > feature detection.
>
> > We'll use similar approach for syscall wrapper detection for
> > BPF_KSYSCALL() BPF-side macro in follow up patch.
>
> > Generally, currently libbpf reserves CONFIG_ prefix for Kconfig values
> > and LINUX_ for virtual libbpf-backed externs. In the future we might
> > extend the set of prefixes that are supported. This can be done without
> > any breaking changes, as currently any __kconfig extern with
> > unrecognized name is rejected.
>
> > For LINUX_xxx externs we support the normal "weak rule": if libbpf
> > doesn't recognize given LINUX_xxx extern but such extern is marked as
> > __weak, it is not rejected and defaults to zero. This follows
> > CONFIG_xxx handling logic and will allow BPF applications to
> > opportunistically use newer libbpf virtual externs without breaking on
> > older libbpf versions unnecessarily.
>
> > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > ---
> > tools/lib/bpf/libbpf.c | 69 +++++++++++++++++++++++++++++-----------
> > tools/lib/bpf/usdt.bpf.h | 16 ++--------
> > 2 files changed, 52 insertions(+), 33 deletions(-)
>
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index cb49408eb298..4bae67767f82 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -1800,11 +1800,18 @@ static bool is_kcfg_value_in_range(const struct
> > extern_desc *ext, __u64 v)
> > static int set_kcfg_value_num(struct extern_desc *ext, void *ext_val,
> > __u64 value)
> > {
> > - if (ext->kcfg.type != KCFG_INT && ext->kcfg.type != KCFG_CHAR) {
> > - pr_warn("extern (kcfg) %s=%llu should be integer\n",
> > + if (ext->kcfg.type != KCFG_INT && ext->kcfg.type != KCFG_CHAR &&
> > + ext->kcfg.type != KCFG_BOOL) {
> > + pr_warn("extern (kcfg) %s=%llu should be integer, char or boolean\n",
> > ext->name, (unsigned long long)value);
> > return -EINVAL;
> > }
> > + if (ext->kcfg.type == KCFG_BOOL && value > 1) {
> > + pr_warn("extern (kcfg) %s=%llu value isn't boolean\n",
> > + ext->name, (unsigned long long)value);
> > + return -EINVAL;
> > +
> > + }
> > if (!is_kcfg_value_in_range(ext, value)) {
> > pr_warn("extern (kcfg) %s=%llu value doesn't fit in %d bytes\n",
> > ext->name, (unsigned long long)value, ext->kcfg.sz);
> > @@ -1870,10 +1877,13 @@ static int
> > bpf_object__process_kconfig_line(struct bpf_object *obj,
> > /* assume integer */
> > err = parse_u64(value, &num);
> > if (err) {
> > - pr_warn("extern (kcfg) %s=%s should be integer\n",
> > - ext->name, value);
> > + pr_warn("extern (kcfg) %s=%s should be integer\n", ext->name, value);
> > return err;
> > }
> > + if (ext->kcfg.type != KCFG_INT && ext->kcfg.type != KCFG_CHAR) {
> > + pr_warn("extern (kcfg) %s=%s should be integer\n", ext->name, value);
> > + return -EINVAL;
> > + }
> > err = set_kcfg_value_num(ext, ext_val, num);
> > break;
> > }
> > @@ -7493,26 +7503,47 @@ static int bpf_object__resolve_externs(struct
> > bpf_object *obj,
> > for (i = 0; i < obj->nr_extern; i++) {
> > ext = &obj->externs[i];
>
> > - if (ext->type == EXT_KCFG &&
> > - strcmp(ext->name, "LINUX_KERNEL_VERSION") == 0) {
> > - void *ext_val = kcfg_data + ext->kcfg.data_off;
> > - __u32 kver = get_kernel_version();
> > + if (ext->type == EXT_KSYM) {
> > + if (ext->ksym.type_id)
> > + need_vmlinux_btf = true;
> > + else
> > + need_kallsyms = true;
> > + continue;
> > + } else if (ext->type == EXT_KCFG) {
> > + void *ext_ptr = kcfg_data + ext->kcfg.data_off;
> > + __u64 value = 0;
> > +
> > + /* Kconfig externs need actual /proc/config.gz */
> > + if (str_has_pfx(ext->name, "CONFIG_")) {
> > + need_config = true;
> > + continue;
> > + }
>
> > - if (!kver) {
> > - pr_warn("failed to get kernel version\n");
> > + /* Virtual kcfg externs are customly handled by libbpf */
> > + if (strcmp(ext->name, "LINUX_KERNEL_VERSION") == 0) {
> > + value = get_kernel_version();
> > + if (!value) {
> > + pr_warn("extern (kcfg) '%s': failed to get kernel version\n",
> > ext->name);
> > + return -EINVAL;
> > + }
> > + } else if (strcmp(ext->name, "LINUX_HAS_BPF_COOKIE") == 0) {
> > + value = kernel_supports(obj, FEAT_BPF_COOKIE);
> > + } else if (!str_has_pfx(ext->name, "LINUX_") || !ext->is_weak) {
> > + /* Currently libbpf supports only CONFIG_ and LINUX_ prefixed
> > + * __kconfig externs, where LINUX_ ones are virtual and filled out
> > + * customly by libbpf (their values don't come from Kconfig).
> > + * If LINUX_xxx variable is not recognized by libbpf, but is marked
> > + * __weak, it defaults to zero value, just like for CONFIG_xxx
> > + * externs.
> > + */
> > + pr_warn("extern (kcfg) '%s': unrecognized virtual extern\n",
> > ext->name);
> > return -EINVAL;
> > }
> > - err = set_kcfg_value_num(ext, ext_val, kver);
> > +
> > + err = set_kcfg_value_num(ext, ext_ptr, value);
> > if (err)
> > return err;
> > - pr_debug("extern (kcfg) %s=0x%x\n", ext->name, kver);
> > - } else if (ext->type == EXT_KCFG && str_has_pfx(ext->name, "CONFIG_"))
> > {
> > - need_config = true;
> > - } else if (ext->type == EXT_KSYM) {
> > - if (ext->ksym.type_id)
> > - need_vmlinux_btf = true;
> > - else
> > - need_kallsyms = true;
> > + pr_debug("extern (kcfg) %s=0x%llx\n", ext->name, (long long)value);
> > } else {
> > pr_warn("unrecognized extern '%s'\n", ext->name);
> > return -EINVAL;
> > diff --git a/tools/lib/bpf/usdt.bpf.h b/tools/lib/bpf/usdt.bpf.h
> > index 4181fddb3687..4f2adc0bd6ca 100644
> > --- a/tools/lib/bpf/usdt.bpf.h
> > +++ b/tools/lib/bpf/usdt.bpf.h
> > @@ -6,7 +6,6 @@
> > #include <linux/errno.h>
> > #include <bpf/bpf_helpers.h>
> > #include <bpf/bpf_tracing.h>
> > -#include <bpf/bpf_core_read.h>
>
> > /* Below types and maps are internal implementation details of libbpf's
> > USDT
> > * support and are subjects to change. Also, bpf_usdt_xxx() API helpers
> > should
> > @@ -30,14 +29,6 @@
> > #ifndef BPF_USDT_MAX_IP_CNT
> > #define BPF_USDT_MAX_IP_CNT (4 * BPF_USDT_MAX_SPEC_CNT)
> > #endif
> > -/* We use BPF CO-RE to detect support for BPF cookie from BPF side. This
> > is
> > - * the only dependency on CO-RE, so if it's undesirable, user can
> > override
> > - * BPF_USDT_HAS_BPF_COOKIE to specify whether to BPF cookie is supported
> > or not.
> > - */
> > -#ifndef BPF_USDT_HAS_BPF_COOKIE
> > -#define BPF_USDT_HAS_BPF_COOKIE \
> > - bpf_core_enum_value_exists(enum bpf_func_id___usdt,
> > BPF_FUNC_get_attach_cookie___usdt)
> > -#endif
>
> > enum __bpf_usdt_arg_type {
> > BPF_USDT_ARG_CONST,
> > @@ -83,15 +74,12 @@ struct {
> > __type(value, __u32);
> > } __bpf_usdt_ip_to_spec_id SEC(".maps") __weak;
>
> > -/* don't rely on user's BPF code to have latest definition of
> > bpf_func_id */
> > -enum bpf_func_id___usdt {
> > - BPF_FUNC_get_attach_cookie___usdt = 0xBAD, /* value doesn't matter */
> > -};
> > +extern const _Bool LINUX_HAS_BPF_COOKIE __kconfig;
>
> Could _Bool be a problem when used by c++? I remember that we can have
> sizeof(bool) != sizeof(_Bool) when compiling with g++. If we were to
> use 'int' instead I'm assuming we'll loose all the niceties of
> KCFG_BOOL?
>
> Or shouldn't be a problem since it's not part of C/C++ api boundary?
I actually don't know if C++ supports "_Bool", but in C, "bool" is an
alias to _Bool. _Bool is *the type* of the boolean. The benefit of
_Bool is that it doesn't require including stdbool.h, while "bool"
itself needs extra header. So I try not to use bool in libbpf BPF API
headers just to avoid extra header dependencies.
But it shouldn't matter as this is BPF-side code, so it has to be
compiled in C mode by Clang/GCC, so _Bool should always be there.
As for the size it seems like it's not even specified by the standard
that sizeof(bool/_Bool) is 1, though it is in practice. I only
remember some very-very-very old versions of Microsoft's Visual C++
having sizeof(bool) == 4, but then they changed that anyways to
sizeof(bool) == 1 (it was many-many years ago, so it might be an
incomplete story).
But either way it doesn't matter, because libbpf will support any
size: 1, 2, 4, 8 and will just set 1 for true, 0 for false, with
correct zero extension to match variable size.
As for bool vs int, no real difference, but it is true/false
conceptually, so seems cleaner to use bool. But using int will work
just fine here as well (you still get 0 or 1 for both, effectively).
>
> > static __always_inline
> > int __bpf_usdt_spec_id(struct pt_regs *ctx)
> > {
> > - if (!BPF_USDT_HAS_BPF_COOKIE) {
> > + if (!LINUX_HAS_BPF_COOKIE) {
> > long ip = PT_REGS_IP(ctx);
> > int *spec_id_ptr;
>
> > --
> > 2.30.2
>
