[PATCH v4 0/2] x86/crash: size the elfcorehdr reservation with NR_CPUS

[Ionut Nechita (Wind River)]

From: Ionut Nechita <ionut.nechita@xxxxxxxxxxxxx>

Hi,

The x86 crash code sizes the elfcorehdr reservation with
CONFIG_NR_CPUS_DEFAULT, which is a Kconfig default and not an upper
bound.  The header itself carries one phdr per possible CPU, so the
reservation is too small whenever NR_CPUS exceeds NR_CPUS_DEFAULT.

Patch 1 sizes it with CONFIG_NR_CPUS instead.  Patch 2 fixes the two
comments that still name NR_CPUS_DEFAULT.

Based on linux-next (next-20260826, base-commit f82a5da2f0496).

Changes since v3:
- Patch 1/2: cut the changelog down to what Dave Hansen suggested.  I
  kept one sentence he had dropped - that kexec_file_load(2) fails with
  -EINVAL - because that is the failure this was reported for and what
  the Fixes: tag addresses; the silent kexec_load(2) truncation is the
  other half.  Happy to trim further if that is still too much.
- Patch 2/2: changelog trimmed the same way.
- No code changes; the diff is byte-identical to v2 and v3.
- Rebased from next-20260825 onto next-20260826.

Reviewers: the code has not moved since v2, so I carried the tags again.

Reproducer, for the archive - single-socket Xeon 6776P, PREEMPT_RT, with
CONFIG_NR_CPUS_DEFAULT=64, CONFIG_NR_CPUS=256, CONFIG_MAXSMP and
CONFIG_MEMORY_HOTPLUG off, CONFIG_CRASH_MAX_MEMORY_RANGES=8192:

- 144 possible CPUs: the buffer overshoots the reservation by
  (144 - 64) * 56 = 4480 bytes, more than the 4096 bytes of page
  padding, and 'kexec -p -s' fails with
  "kexec_file_load failed: Invalid argument"
- 72 possible CPUs (reduced via firmware): the overshoot is
  (72 - 64) * 56 = 448 bytes, still absorbed by the page rounding, and
  the load succeeds

The two follow-ups Sourabh Jain raised on the v2 thread - the
add_segment_phys_virt() bufsz truncation in kexec-tools, and having
crash_load_segments() skip the kbuf.memsz update when the prepared
buffer is already larger - are still deliberately out of scope here.

v1: https://lore.kernel.org/lkml/20260812170433.533845-1-ionut.nechita@xxxxxxxxxxxxx/
v2: https://lore.kernel.org/lkml/20260825075043.42041-1-ionut.nechita@xxxxxxxxxxxxx/
v3: https://lore.kernel.org/lkml/20260826073527.21487-1-ionut.nechita@xxxxxxxxxxxxx/

Ionut Nechita (2):
  x86/crash: reserve elfcorehdr for CONFIG_NR_CPUS, not
    CONFIG_NR_CPUS_DEFAULT
  crash: update stale NR_CPUS_DEFAULT references in elfcorehdr sizing
    docs

 arch/x86/kernel/crash.c | 6 +++---
 kernel/Kconfig.kexec    | 2 +-
 kernel/crash_core.c     | 2 +-
 3 files changed, 5 insertions(+), 5 deletions(-)


base-commit: f82a5da2f04960df9fb57489992d03dd5e64ec6f
--
2.55.0