2022-07-12 23:53:16

by James Hilliard

Subject: [PATCH v3] bpf/scripts: Generate GCC compatible helpers

The current bpf_helper_defs.h helpers are llvm specific and don't work
correctly with gcc.

GCC appears to required kernel helper funcs to have the following
attribute set: __attribute__((kernel_helper(NUM)))

Generate gcc compatible headers based on the format in bpf-helpers.h.

This generates GCC/Clang compatible helpers, for example:
/* Helper macro for GCC/Clang compatibility */
#define NOARG
#if __GNUC__ && !__clang__
#define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \
ret_type ret_star name(__VA_ARGS__) __attribute__((kernel_helper(num)));
#else
#define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \
static ret_type ret_star(*name)(__VA_ARGS__) = (void *) num;
#endif

BPF_HELPER_DEF(1, *, void, bpf_map_lookup_elem, void *map, const void *key)

BPF_HELPER_DEF(2, NOARG, long, bpf_map_update_elem, void *map, const void *key, const void *value, __u64 flags)

See:
https://github.com/gcc-mirror/gcc/blob/releases/gcc-12.1.0/gcc/config/bpf/bpf-helpers.h#L24-L27

This fixes the following build error:
error: indirect call in function, which are not supported by eBPF

Signed-off-by: James Hilliard <[email protected]>
---
Changes v2 -> v3:
- use a conditional helper macro
Changes v1 -> v2:
- more details in commit log
---
scripts/bpf_doc.py | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/scripts/bpf_doc.py b/scripts/bpf_doc.py
index a0ec321469bd..45f51ff1318c 100755
--- a/scripts/bpf_doc.py
+++ b/scripts/bpf_doc.py
@@ -717,6 +717,16 @@ class PrinterHelpers(Printer):
header = '''\
/* This is auto-generated file. See bpf_doc.py for details. */

+/* Helper macro for GCC/Clang compatibility */
+#define NOARG
+#if __GNUC__ && !__clang__
+#define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \\
+ret_type ret_star name(__VA_ARGS__) __attribute__((kernel_helper(num)));
+#else
+#define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \\
+static ret_type ret_star(*name)(__VA_ARGS__) = (void *) num;
+#endif
+
/* Forward declarations of BPF structs */'''

print(header)
@@ -746,6 +756,11 @@ class PrinterHelpers(Printer):
return
self.seen_helpers.add(proto['name'])

+ if proto['ret_star']:
+ ret_star = proto['ret_star']
+ else:
+ ret_star = 'NOARG'
+
print('/*')
print(" * %s" % proto['name'])
print(" *")
@@ -762,8 +777,8 @@ class PrinterHelpers(Printer):
print(' *{}{}'.format(' \t' if line else '', line))

print(' */')
- print('static %s %s(*%s)(' % (self.map_type(proto['ret_type']),
- proto['ret_star'], proto['name']), end='')
+ print('BPF_HELPER_DEF(%d, %s, %s, %s, ' % (len(self.seen_helpers),
+ ret_star, self.map_type(proto['ret_type']), proto['name']), end='')
comma = ''
for i, a in enumerate(proto['args']):
t = a['type']
@@ -781,7 +796,7 @@ class PrinterHelpers(Printer):
comma = ', '
print(one_arg, end='')

- print(') = (void *) %d;' % len(self.seen_helpers))
+ print(')')
print('')

###############################################################################
--
2.34.1

2022-07-13 00:00:36

by Alexei Starovoitov

Subject: Re: [PATCH v3] bpf/scripts: Generate GCC compatible helpers

On Tue, Jul 12, 2022 at 4:26 PM James Hilliard
<[email protected]> wrote:
>
> The current bpf_helper_defs.h helpers are llvm specific and don't work
> correctly with gcc.
>
> GCC appears to required kernel helper funcs to have the following
> attribute set: __attribute__((kernel_helper(NUM)))
>
> Generate gcc compatible headers based on the format in bpf-helpers.h.
>
> This generates GCC/Clang compatible helpers, for example:
> /* Helper macro for GCC/Clang compatibility */
> #define NOARG
> #if __GNUC__ && !__clang__
> #define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \
> ret_type ret_star name(__VA_ARGS__) __attribute__((kernel_helper(num)));
> #else
> #define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \
> static ret_type ret_star(*name)(__VA_ARGS__) = (void *) num;
> #endif
>
> BPF_HELPER_DEF(1, *, void, bpf_map_lookup_elem, void *map, const void *key)
>
> BPF_HELPER_DEF(2, NOARG, long, bpf_map_update_elem, void *map, const void *key, const void *value, __u64 flags)
>
> See:
> https://github.com/gcc-mirror/gcc/blob/releases/gcc-12.1.0/gcc/config/bpf/bpf-helpers.h#L24-L27
>
> This fixes the following build error:
> error: indirect call in function, which are not supported by eBPF
>
> Signed-off-by: James Hilliard <[email protected]>
> ---
> Changes v2 -> v3:
> - use a conditional helper macro
> Changes v1 -> v2:
> - more details in commit log
> ---
> scripts/bpf_doc.py | 21 ++++++++++++++++++---
> 1 file changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/scripts/bpf_doc.py b/scripts/bpf_doc.py
> index a0ec321469bd..45f51ff1318c 100755
> --- a/scripts/bpf_doc.py
> +++ b/scripts/bpf_doc.py
> @@ -717,6 +717,16 @@ class PrinterHelpers(Printer):
> header = '''\
> /* This is auto-generated file. See bpf_doc.py for details. */
>
> +/* Helper macro for GCC/Clang compatibility */
> +#define NOARG
> +#if __GNUC__ && !__clang__
> +#define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \\
> +ret_type ret_star name(__VA_ARGS__) __attribute__((kernel_helper(num)));
> +#else
> +#define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \\
> +static ret_type ret_star(*name)(__VA_ARGS__) = (void *) num;
> +#endif

Nack for the reasons stated earlier.