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libbpf: Handle GCC noreturn-turned-volatile quirk 

Handle a GCC quirk of emitting extra volatile modifier in DWARF (and
subsequently preserved in BTF by pahole) for function pointers marked as
__attribute__((noreturn)). This was the way to mark such functions before GCC
2.5 added noreturn attribute. Drop such func_proto modifiers, similarly to how
it's done for array (also to handle GCC quirk/bug).

Such volatile attribute is emitted by GCC only, so existing selftests can't
express such test. Simple repro is like this (compiled with GCC + BTF
generated by pahole):

  struct my_struct {
      void __attribute__((noreturn)) (*fn)(int);
  };
  struct my_struct a;

Without this fix, output will be:

struct my_struct {
    voidvolatile  (*fn)(int);
};

With the fix:

struct my_struct {
    void (*fn)(int);
};

Fixes: 351131b51c ("libbpf: add btf_dump API for BTF-to-C conversion")
Reported-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Link: https://lore.kernel.org/bpf/20200610052335.2862559-1-andriin@fb.com
This commit is contained in:
Andrii Nakryiko 2020-06-09 22:23:35 -07:00 committed by Daniel Borkmann
parent 8ca8d4a841
commit 32022fd97e
1 changed files with 24 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

33
tools/lib/bpf/btf_dump.c
View File

@ -1137,6 +1137,20 @@ static void btf_dump_emit_mods(struct btf_dump *d, struct id_stack *decl_stack)
}
}
static void btf_dump_drop_mods(struct btf_dump *d, struct id_stack *decl_stack)
{
const struct btf_type *t;
__u32 id;
while (decl_stack->cnt) {
id = decl_stack->ids[decl_stack->cnt - 1];
t = btf__type_by_id(d->btf, id);
if (!btf_is_mod(t))
return;
decl_stack->cnt--;
}
}
static void btf_dump_emit_name(const struct btf_dump *d,
const char *name, bool last_was_ptr)
{
@ -1235,14 +1249,7 @@ static void btf_dump_emit_type_chain(struct btf_dump *d,
* a const/volatile modifier for array, so we are
* going to silently skip them here.
*/
while (decls->cnt) {
next_id = decls->ids[decls->cnt - 1];
next_t = btf__type_by_id(d->btf, next_id);
if (btf_is_mod(next_t))
decls->cnt--;
else
break;
}
btf_dump_drop_mods(d, decls);
if (decls->cnt == 0) {
btf_dump_emit_name(d, fname, last_was_ptr);
@ -1270,7 +1277,15 @@ static void btf_dump_emit_type_chain(struct btf_dump *d,
__u16 vlen = btf_vlen(t);
int i;
btf_dump_emit_mods(d, decls);
/*
* GCC emits extra volatile qualifier for
* __attribute__((noreturn)) function pointers. Clang
* doesn't do it. It's a GCC quirk for backwards
* compatibility with code written for GCC <2.5. So,
* similarly to extra qualifiers for array, just drop
* them, instead of handling them.
*/
btf_dump_drop_mods(d, decls);
if (decls->cnt) {
btf_dump_printf(d, " (");
btf_dump_emit_type_chain(d, decls, fname, lvl);