Messages in this thread Patch in this message | | | Date | Fri, 13 Nov 2020 11:31:59 +0100 | From | Jessica Yu <> | Subject | Re: [PATCH v4 bpf-next 4/5] bpf: load and verify kernel module BTFs |
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+++ 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
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