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btf: Avoid weak external references 

If the BTF code is enabled in the build configuration, the start/stop
BTF markers are guaranteed to exist. Only when CONFIG_DEBUG_INFO_BTF=n,
the references in btf_parse_vmlinux() will remain unsatisfied, relying
on the weak linkage of the external references to avoid breaking the
build.

Avoid GOT based relocations to these markers in the final executable by
dropping the weak attribute and instead, make btf_parse_vmlinux() return
ERR_PTR(-ENOENT) directly if CONFIG_DEBUG_INFO_BTF is not enabled to
begin with.  The compiler will drop any subsequent references to
__start_BTF and __stop_BTF in that case, allowing the link to succeed.

Note that Clang will notice that taking the address of __start_BTF can
no longer yield NULL, so testing for that condition becomes unnecessary.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20240415162041.2491523-8-ardb+git@google.com
This commit is contained in:
Ard Biesheuvel 2024-04-15 18:20:45 +02:00 committed by Daniel Borkmann
parent 4d4992ff58
commit fc5eb4a84e
2 changed files with 8 additions and 5 deletions
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7
kernel/bpf/btf.c
View File

@ -5642,8 +5642,8 @@ errout_free:
return ERR_PTR(err);
}
extern char __weak __start_BTF[];
extern char __weak __stop_BTF[];
extern char __start_BTF[];
extern char __stop_BTF[];
extern struct btf *btf_vmlinux;
#define BPF_MAP_TYPE(_id, _ops)
@ -5971,6 +5971,9 @@ struct btf *btf_parse_vmlinux(void)
struct btf *btf = NULL;
int err;
if (!IS_ENABLED(CONFIG_DEBUG_INFO_BTF))
return ERR_PTR(-ENOENT);
env = kzalloc(sizeof(*env), GFP_KERNEL | __GFP_NOWARN);
if (!env)
return ERR_PTR(-ENOMEM);

6
kernel/bpf/sysfs_btf.c
View File

@ -9,8 +9,8 @@
#include <linux/sysfs.h>
/* See scripts/link-vmlinux.sh, gen_btf() func for details */
extern char __weak __start_BTF[];
extern char __weak __stop_BTF[];
extern char __start_BTF[];
extern char __stop_BTF[];
static ssize_t
btf_vmlinux_read(struct file *file, struct kobject *kobj,
@ -32,7 +32,7 @@ static int __init btf_vmlinux_init(void)
{
bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
if (!__start_BTF || bin_attr_btf_vmlinux.size == 0)
if (bin_attr_btf_vmlinux.size == 0)
return 0;
btf_kobj = kobject_create_and_add("btf", kernel_kobj);