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SubjectRe: [PATCH v2][RFC] kdump: append kernel build-id string to VMCOREINFO
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Thanks Andrew.  See inline.

On 6/17/2020 12:45 PM, Andrew Morton wrote:
> On Wed, 10 Jun 2020 21:27:52 -0700 Vijay Balakrishna <vijayb@linux.microsoft.com> wrote:
>
>> Make kernel GNU build-id available in VMCOREINFO. Having
>> build-id in VMCOREINFO facilitates presenting appropriate kernel
>> namelist image with debug information file to kernel crash dump
>> analysis tools. Currently VMCOREINFO lacks uniquely identifiable
>> key for crash analysis automation.
>>
>> Regarding if this patch is necessary or matching of linux_banner
>> and OSRELEASE in VMCOREINFO employed by crash(8) meets the
>> need -- IMO, build-id approach more foolproof, in most instances it
>> is a cryptographic hash generated using internal code/ELF bits unlike
>> kernel version string upon which linux_banner is based that is
>> external to the code. I feel each is intended for a different purpose.
>> Also OSRELEASE is not suitable when two different kernel builds
>> from same version with different features enabled.
>>
>> Currently for most linux (and non-linux) systems build-id can be
>> extracted using standard methods for file types such as user mode crash
>> dumps, shared libraries, loadable kernel modules etc., This is an
>> exception for linux kernel dump. Having build-id in VMCOREINFO brings
>> some uniformity for automation tools.
>>
>> ...
>>
>> --- a/kernel/crash_core.c
>> +++ b/kernel/crash_core.c
>> @@ -11,6 +11,8 @@
>> #include <asm/page.h>
>> #include <asm/sections.h>
>>
>> +#include <crypto/sha.h>
>> +
>> /* vmcoreinfo stuff */
>> unsigned char *vmcoreinfo_data;
>> size_t vmcoreinfo_size;
>> @@ -376,6 +378,53 @@ phys_addr_t __weak paddr_vmcoreinfo_note(void)
>> }
>> EXPORT_SYMBOL(paddr_vmcoreinfo_note);
>>
>> +#define NOTES_SIZE (&__stop_notes - &__start_notes)
>> +#define BUILD_ID_MAX SHA1_DIGEST_SIZE
>> +#define NT_GNU_BUILD_ID 3
>> +
>> +struct elf_note_section {
>> + struct elf_note n_hdr;
>> + u8 n_data[];
>> +};
>> +
>> +/*
>> + * Add build ID from .notes section as generated by the GNU ld(1)
>> + * or LLVM lld(1) --build-id option.
>> + */
>> +static void add_build_id_vmcoreinfo(void)
>> +{
>> + char build_id[BUILD_ID_MAX * 2 + 1];
>> + int n_remain = NOTES_SIZE;
>> +
>> + while (n_remain >= sizeof(struct elf_note)) {
>> + const struct elf_note_section *note_sec =
>> + &__start_notes + NOTES_SIZE - n_remain;
>> + const u32 n_namesz = note_sec->n_hdr.n_namesz;
>> +
>> + if (note_sec->n_hdr.n_type == NT_GNU_BUILD_ID &&
>> + n_namesz != 0 &&
>> + !strcmp((char *)&note_sec->n_data[0], "GNU")) {
>
> Is it guaranteed that n_data[] is null-terminated?

From ELF(5) "Notes (Nhdr)" section:

> n_namesz The length of the name field in bytes. The contents will immediately follow this note in memory. The name is null terminated. For
> example, if the name is "GNU", then n_namesz will be set to 4.

Vijay

>
>> + if (note_sec->n_hdr.n_descsz <= BUILD_ID_MAX) {
>> + const u32 n_descsz = note_sec->n_hdr.n_descsz;
>> + const u8 *s = &note_sec->n_data[n_namesz];
>> +
>> + s = PTR_ALIGN(s, 4);
>> + bin2hex(build_id, s, n_descsz);
>> + build_id[2 * n_descsz] = '\0';
>> + VMCOREINFO_BUILD_ID(build_id);
>> + return;
>> + }
>> + pr_warn("Build ID is too large to include in vmcoreinfo: %u > %u\n",
>> + note_sec->n_hdr.n_descsz,
>> + BUILD_ID_MAX);
>> + return;
>> + }
>> + n_remain -= sizeof(struct elf_note) +
>> + ALIGN(note_sec->n_hdr.n_namesz, 4) +
>> + ALIGN(note_sec->n_hdr.n_descsz, 4);
>> + }
>> +}
>> +
>> static int __init crash_save_vmcoreinfo_init(void)
>> {
>> vmcoreinfo_data = (unsigned char *)get_zeroed_page(GFP_KERNEL);

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