From: Jessica Yu <jeyu@kernel.org>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Andrii Nakryiko <andrii@kernel.org>, bpf <bpf@vger.kernel.org>,
Networking <netdev@vger.kernel.org>,
Alexei Starovoitov <ast@fb.com>,
Daniel Borkmann <daniel@iogearbox.net>,
Kernel Team <kernel-team@fb.com>,
open list <linux-kernel@vger.kernel.org>,
"Rafael J. Wysocki" <rafael@kernel.org>,
Arnaldo Carvalho de Melo <acme@redhat.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH v4 bpf-next 4/5] bpf: load and verify kernel module BTFs
Date: Fri, 13 Nov 2020 11:31:59 +0100 [thread overview]
Message-ID: <20201113103158.GA30836@linux-8ccs> (raw)
In-Reply-To: <CAEf4BzZbKRgWhLD6KFOwJU8DDns9oufroBShczM9KqODCqbEPA@mail.gmail.com>
+++ Andrii Nakryiko [11/11/20 12:11 -0800]:
>On Wed, Nov 11, 2020 at 2:13 AM Jessica Yu <jeyu@kernel.org> wrote:
>>
>> +++ Andrii Nakryiko [09/11/20 17:19 -0800]:
>> [snipped]
>> >diff --git a/kernel/module.c b/kernel/module.c
>> >index a4fa44a652a7..f2996b02ab2e 100644
>> >--- a/kernel/module.c
>> >+++ b/kernel/module.c
>> >@@ -380,6 +380,35 @@ static void *section_objs(const struct load_info *info,
>> > return (void *)info->sechdrs[sec].sh_addr;
>> > }
>> >
>> >+/* Find a module section: 0 means not found. Ignores SHF_ALLOC flag. */
>> >+static unsigned int find_any_sec(const struct load_info *info, const char *name)
>> >+{
>> >+ unsigned int i;
>> >+
>> >+ for (i = 1; i < info->hdr->e_shnum; i++) {
>> >+ Elf_Shdr *shdr = &info->sechdrs[i];
>> >+ if (strcmp(info->secstrings + shdr->sh_name, name) == 0)
>> >+ return i;
>> >+ }
>> >+ return 0;
>> >+}
>> >+
>> >+/*
>> >+ * Find a module section, or NULL. Fill in number of "objects" in section.
>> >+ * Ignores SHF_ALLOC flag.
>> >+ */
>> >+static __maybe_unused void *any_section_objs(const struct load_info *info,
>> >+ const char *name,
>> >+ size_t object_size,
>> >+ unsigned int *num)
>> >+{
>> >+ unsigned int sec = find_any_sec(info, name);
>> >+
>> >+ /* Section 0 has sh_addr 0 and sh_size 0. */
>> >+ *num = info->sechdrs[sec].sh_size / object_size;
>> >+ return (void *)info->sechdrs[sec].sh_addr;
>> >+}
>> >+
>>
>> Hm, I see this patchset has already been applied to bpf-next, but I
>> guess that doesn't preclude any follow-up patches :-)
>
>Of course!
>
>>
>> I am not a huge fan of the code duplication here, and also the fact
>> that they're only called in one place. any_section_objs() and
>> find_any_sec() are pretty much identical to section_objs() and
>> find_sec(), other than the fact the former drops the SHF_ALLOC check.
>
>Right, but the alternative was to add a new flag to existing
>section_objs() and find_sec() functions, which would cause much more
>code churn for no good reason (besides saving some trivial code
>duplication). And those true/false flags are harder to read in code
>anyways.
That's true, all fair points. I thought there was the possibility to
avoid the code duplication if .BTF were also set to SHF_ALLOC, but I
see for reasons you explained below it is more trouble than it's worth.
>>
>> Moreover, since it appears that the ".BTF" section is not marked
>> SHF_ALLOC, I think this will leave mod->btf_data as a dangling pointer
>> after the module is done loading and the module's load_info has been
>> deallocated, since SHF_ALLOC sections are not allocated nor copied to
>> the module's final location in memory.
>
>I can make sure that we also reset the btf_data pointer back to NULL,
>if that's a big concern.
It's not a terribly huge concern, since mod->btf_data is only accessed
in the btf coming notifier at the moment, but it's probably best to at
least not advertise it as a valid pointer anymore after the module is
done loading. We do some pointer and section size cleanup at the end
of do_init_module() for sections that are deallocated at the end of
module load (starting where init_layout.base is reset to NULL),
we could just tack on mod->btf_data = NULL there as well.
>>
>> Why not simply mark the ".BTF" section in the module SHF_ALLOC? We
>> already do some sh_flags rewriting in rewrite_section_headers(). Then
>> the module loader knows to keep the section in memory and you can use
>> section_objs(). And since the .BTF section stays in module memory,
>> that might save you the memcpy() to btf->data in btf_parse_module()
>> (unless that is still needed for some reason).
>
>Wasn't aware about rewrite_section_headers() manipulations. Are you
>suggesting to just add SHF_ALLOC there for the .BTF section from the
>kernel side? I guess that would work, but won't avoid memory copy (so
>actually would waste kernel memory, if I understand correctly). The
>reason being that the module's BTF is registered as an independently
>ref-counted BTF object, which could be held past the kernel module
>being unloaded. So I can't directly reference module's .BTF data
>anyways.
Ah OK, I was not aware that the section could be held past the module
being unloaded. Then yeah, it would be a memory waste to keep them in
memory if they are being memcpy'd anyway. Thanks for clarifying!
Jessica
next prev parent reply other threads:[~2020-11-13 10:32 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-10 1:19 [PATCH v4 bpf-next 0/5] Integrate kernel module BTF support Andrii Nakryiko
2020-11-10 1:19 ` [PATCH v4 bpf-next 1/5] bpf: add in-kernel split " Andrii Nakryiko
2020-11-10 17:49 ` Song Liu
2020-11-10 18:31 ` Andrii Nakryiko
2020-11-10 20:14 ` Song Liu
2020-11-10 1:19 ` [PATCH v4 bpf-next 2/5] bpf: assign ID to vmlinux BTF and return extra info for BTF in GET_OBJ_INFO Andrii Nakryiko
2020-11-10 20:24 ` Song Liu
2020-11-10 1:19 ` [PATCH v4 bpf-next 3/5] kbuild: build kernel module BTFs if BTF is enabled and pahole supports it Andrii Nakryiko
2020-11-11 1:04 ` Song Liu
2020-11-16 19:55 ` Allan, Bruce W
2020-11-16 20:34 ` Andrii Nakryiko
2020-11-16 21:24 ` Jakub Kicinski
2020-11-16 22:35 ` Andrii Nakryiko
2021-11-26 15:16 ` Fabian Grünbichler
2021-12-08 0:08 ` Andrii Nakryiko
2020-11-17 3:44 ` David Ahern
2020-11-10 1:19 ` [PATCH v4 bpf-next 4/5] bpf: load and verify kernel module BTFs Andrii Nakryiko
2020-11-11 7:11 ` kernel test robot
2020-11-11 10:13 ` Jessica Yu
2020-11-11 20:11 ` Andrii Nakryiko
2020-11-13 10:31 ` Jessica Yu [this message]
2020-11-13 18:54 ` Andrii Nakryiko
2020-11-10 1:19 ` [PATCH v4 bpf-next 5/5] tools/bpftool: add support for in-kernel and named BTF in `btf show` Andrii Nakryiko
2020-11-11 1:15 ` Song Liu
2020-11-11 4:19 ` Andrii Nakryiko
2020-11-11 7:44 ` Song Liu
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