Re: [PATCH v2][RFC] kdump: append kernel build-id string to VMCOREINFO

[Andrew Morton <akpm@linux-foundation.org>]

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?

> +			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);