Subject: Re: [PATCH v4 bpf-next 4/5] bpf: load and verify kernel module BTFs

From: Jessica Yu <[email-masked]>
Date: Fri, 13 Nov 2020 11:31:59 +0100

+++ Andrii Nakryiko [11/11/20 12:11 -0800]:
>On Wed, Nov 11, 2020 at 2:13 AM Jessica Yu <[email-masked]> 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