Kernel Crash Dump (kdump) broken with 6.5

[Mahesh J Salgaonkar]

On 2023-07-18 23:19:23 Tue, Michael Ellerman wrote:
> Mahesh J Salgaonkar <mahesh at linux.ibm.com> writes:
> > On 2023-07-17 20:15:53 Mon, Sachin Sant wrote:
> >> Kdump seems to be broken with 6.5 for ppc64le.
> >> 
> >> [ 14.200412] systemd[1]: Starting dracut pre-pivot and cleanup hook...
> >> [[0;32m OK [0m] Started dracut pre-pivot and cleanup hook.
> >> Starting Kdump Vmcore Save Service...
> >> [ 14.231669] systemd[1]: Started dracut pre-pivot and cleanup hook.
> >> [ 14.231801] systemd[1]: Starting Kdump Vmcore Save Service...
> >> [ 14.341035] kdump.sh[297]: kdump: saving to /sysroot//var/crash//127.0.0.1-2023-07-14-13:32:34/
> >> [ 14.350053] EXT4-fs (sda2): re-mounted e971a335-1ef8-4295-ab4e-3940f28e53fc r/w. Quota mode: none.
> >> [ 14.345979] kdump.sh[297]: kdump: saving vmcore-dmesg.txt to /sysroot//var/crash//127.0.0.1-2023-07-14-13:32:34/
> >> [ 14.348742] kdump.sh[331]: Cannot open /proc/vmcore: No such file or directory
> >> [ 14.348845] kdump.sh[297]: kdump: saving vmcore-dmesg.txt failed
> >> [ 14.349014] kdump.sh[297]: kdump: saving vmcore
> >> [ 14.443422] kdump.sh[332]: open_dump_memory: Can't open the dump memory(/proc/vmcore). No such file or directory
> >> [ 14.456413] kdump.sh[332]: makedumpfile Failed.
> >> [ 14.456662] kdump.sh[297]: kdump: saving vmcore failed, _exitcode:1
> >> [ 14.456822] kdump.sh[297]: kdump: saving the /run/initramfs/kexec-dmesg.log to /sysroot//var/crash//127.0.0.1-2023-07-14-13:32:34/
> >> [ 14.487002] kdump.sh[297]: kdump: saving vmcore failed
> >> [[0;1;31mFAILED[0m] Failed to start Kdump Vmcore Save Service.
> >
> > Thanks Sachin for catching this.
> >
> >> 
> >> 6.4 was good. Git bisect points to following patch
> >> 
> >> commit 606787fed7268feb256957872586370b56af697a
> >>     powerpc/64s: Remove support for ELFv1 little endian userspace
> >> 
> >> Reverting this patch allows a successful capture of vmcore.
> >> 
> >> Does this change require any corresponding change to kdump
> >> and/or kexec tools?
> >
> > Need to investigate that. It looks like vmcore_elf64_check_arch(&ehdr)
> > check from fs/proc/vmcore.c is failing after above commit.
> >
> > static int __init parse_crash_elf64_headers(void)
> > {
> > [...]
> >
> >         /* Do some basic Verification. */
> >         if (memcmp(ehdr.e_ident, ELFMAG, SELFMAG) != 0 ||
> >                 (ehdr.e_type != ET_CORE) ||
> >                 !vmcore_elf64_check_arch(&ehdr) ||
> > [...]
> 
> Where vmcore_elf64_check_arch() calls elf_check_arch(), which was
> modified by the commit, so that makes sense.
> 
> > It looks like ehdr->e_flags are not set properly while generating vmcore
> > ELF header. I see that in kexec_file_load, ehdr->e_flags left set to 0
> > irrespective of IS_ENABLED(CONFIG_PPC64_ELF_ABI_V2) is true or false.
> 
> Does initialising it in crash_prepare_elf64_headers() fix the issue?

Yes, the bellow change fixes the issue. Can't use
IS_ENABLED(CONFIG_PPC64_ELF_ABI_V2)i check in common code. I see that
fs/proc/kcore.c uses ELF_CORE_EFLAGS to set e_flags. Will send out
formal patch.

>From 2d12fe7dff5dfe9035a75b1fe8d7da7da3000b90 Mon Sep 17 00:00:00 2001
From: Mahesh Salgaonkar <mahesh at linux.ibm.com>
Date: Wed, 19 Jul 2023 20:36:37 +0530
Subject: [PATCH] kdump fix

Signed-off-by: Mahesh Salgaonkar <mahesh at linux.ibm.com>
---
 kernel/kexec_file.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index 881ba0d1714cc..be51560509128 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -1279,6 +1279,7 @@ int crash_prepare_elf64_headers(struct crash_mem *mem, int need_kernel_map,
 	ehdr->e_phoff = sizeof(Elf64_Ehdr);
 	ehdr->e_ehsize = sizeof(Elf64_Ehdr);
 	ehdr->e_phentsize = sizeof(Elf64_Phdr);
+	ehdr->e_flags = ELF_CORE_EFLAGS;
 
 	/* Prepare one phdr of type PT_NOTE for each present CPU */
 	for_each_present_cpu(cpu) {
-- 
2.41.0

-- 
Mahesh J Salgaonkar