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    Subject[PATCH 5.4 251/314] libbpf: Handle GCC noreturn-turned-volatile quirk
    Date
    From: Andrii Nakryiko <andriin@fb.com>

    [ Upstream commit 32022fd97ed34f6812802bf1288db27c313576f4 ]

    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: 351131b51c7a ("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
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    tools/lib/bpf/btf_dump.c | 33 ++++++++++++++++++++++++---------
    1 file changed, 24 insertions(+), 9 deletions(-)

    diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
    index 87f27e2664c5d..d9e386b8f47ed 100644
    --- a/tools/lib/bpf/btf_dump.c
    +++ b/tools/lib/bpf/btf_dump.c
    @@ -1141,6 +1141,20 @@ 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)
    {
    @@ -1239,14 +1253,7 @@ 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);
    @@ -1274,7 +1281,15 @@ 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);
    --
    2.25.1


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