yonghong-song added a comment.
@aprantl The following example shows the difference between gcc and clang
w.r.t. reserved names.
[$ ~/tmp3] cat t.c
extern void *__bpf_kptr_new(int, int, void *)
__attribute__((section(".ksyms")));
#define bpf_kptr_new(x) __bpf_kptr_new(x, 0, 0)
struct foo {
int data;
};
int main(void)
{
struct foo *f;
f = bpf_kptr_new(0);
return f->data;
}
[$ ~/tmp3] clang -target bpf -O2 -g -c t.c
[$ ~/tmp3] llvm-dwarfdump t.o | grep bpf_kptr_new
[$ ~/tmp3] gcc -O2 -g -c t.c
[$ ~/tmp3] llvm-dwarfdump t.o | grep bpf_kptr_new
DW_AT_abstract_origin (0x000000a3 "__bpf_kptr_new")
DW_AT_linkage_name ("__bpf_kptr_new")
DW_AT_name ("__bpf_kptr_new")
[$ ~/tmp3]
If not using reserved name, the function will appear in debuginfo/dwarf:
[$ ~/tmp3] cat t1.c
extern void *bpf_kptr_new_(int, int, void *)
__attribute__((section(".ksyms")));
#define bpf_kptr_new(x) bpf_kptr_new_(x, 0, 0)
struct foo {
int data;
};
int main(void)
{
struct foo *f;
f = bpf_kptr_new(0);
return f->data;
}
[$ ~/tmp3] clang -target bpf -O2 -g -c t1.c
[$ ~/tmp3] llvm-dwarfdump t1.o | grep bpf_kptr_new
DW_AT_name ("bpf_kptr_new_")
[$~/tmp3] gcc -O2 -g -c t1.c
[$ ~/tmp3] llvm-dwarfdump t1.o | grep bpf_kptr_new
DW_AT_abstract_origin (0x000000a3 "bpf_kptr_new_")
DW_AT_linkage_name ("bpf_kptr_new_")
DW_AT_name ("bpf_kptr_new_")
[$ ~/tmp3]
In BPF, we need the above extern function in debuginfo. This happens when a
bpf program is calling a function defined in kernel. For example,
in kernel we have function
int tcp_drop(struct sk_buff *skb, ...) { ...
}
/* allow tcp_drop() function accessible to bpf programs */
In bpf program, we could have
extern int tcp_drop(struct sk_buff *sbk, ...);
BPF_PROG(...) {
tcp_drop(...)
BPF verifier will need to check whether the signature of tcp_drop() in bpf
program matched the one in kernel or not. In kernel, we DO have some global
functions with '__' prefix in the function name. Although no reserved function
name works, we want all legal names working so user won't be surprised why some
kernel functions are not available to them.
}
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D136041/new/
https://reviews.llvm.org/D136041
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