Message ID | 20200610052335.2862559-1-andriin@fb.com |
---|---|
State | Accepted |
Delegated to: | BPF Maintainers |
Headers | show |
Series | [bpf] libbpf: handle GCC noreturn-turned-volatile quirk | expand |
On Tue, Jun 09, 2020 at 10:23:35PM -0700, Andrii Nakryiko wrote: > 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); > }; > > Reported-by: Jean-Philippe Brucker <jean-philippe@linaro.org> > Fixes: 351131b51c7a ("libbpf: add btf_dump API for BTF-to-C conversion") > Signed-off-by: Andrii Nakryiko <andriin@fb.com> Thanks, this fixes the issue I was seeing with the arm64 vmlinux Tested-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
On 6/10/20 7:23 AM, Andrii Nakryiko wrote: > 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); > }; > > Reported-by: Jean-Philippe Brucker <jean-philippe@linaro.org> > Fixes: 351131b51c7a ("libbpf: add btf_dump API for BTF-to-C conversion") > Signed-off-by: Andrii Nakryiko <andriin@fb.com> Applied, thanks!
diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c index de07e559a11d..bbb430317260 100644 --- a/tools/lib/bpf/btf_dump.c +++ b/tools/lib/bpf/btf_dump.c @@ -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);
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); }; Reported-by: Jean-Philippe Brucker <jean-philippe@linaro.org> Fixes: 351131b51c7a ("libbpf: add btf_dump API for BTF-to-C conversion") Signed-off-by: Andrii Nakryiko <andriin@fb.com> --- tools/lib/bpf/btf_dump.c | 33 ++++++++++++++++++++++++--------- 1 file changed, 24 insertions(+), 9 deletions(-)