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Wed, 26 Aug 2026 07:35:48 +0000 From: "Ionut Nechita (Wind River)" To: x86@kernel.org, kexec@lists.infradead.org Cc: tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, akpm@linux-foundation.org, baoquan.he@linux.dev, rppt@kernel.org, pasha.tatashin@soleen.com, pratyush@kernel.org, ruirui.yang@linux.dev, eric.devolder@oracle.com, hbathini@linux.ibm.com, sourabhjain@linux.ibm.com, ruanjinjie@huawei.com, include@grrlz.net, linux-kernel@vger.kernel.org Subject: [PATCH v3 1/2] x86/crash: reserve elfcorehdr for CONFIG_NR_CPUS, not CONFIG_NR_CPUS_DEFAULT Date: Wed, 26 Aug 2026 10:35:26 +0300 Message-ID: <20260826073527.21487-2-ionut.nechita@windriver.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260826073527.21487-1-ionut.nechita@windriver.com> References: <20260826073527.21487-1-ionut.nechita@windriver.com> Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: VI1PR0102CA0108.eurprd01.prod.exchangelabs.com (2603:10a6:803:15::49) To 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Nechita kexec_file_load(2) fails with -EINVAL when loading a crash kernel on a machine whose number of possible CPUs exceeds CONFIG_NR_CPUS_DEFAULT. With CONFIG_CRASH_HOTPLUG=3Dy the elfcorehdr segment is over-allocated so it can be updated in place on CPU/memory hotplug. On the !CONFIG_MEMORY_HOTPLUG path, crash_load_segments() sizes that reservation as: ret =3D crash_prepare_headers(..., &kbuf.bufsz, &pnum); ... pnum +=3D 2 + CONFIG_NR_CPUS_DEFAULT; The value that lands in @pnum is crash_prepare_headers()'s @nr_mem_ranges out parameter, i.e. cmem->nr_ranges - the number of memory ranges only, not a phdr count. The header that crash_prepare_elf64_headers() actually builds adds one phdr per *possible* CPU on top of those ranges: nr_phdr =3D nr_cpus + 1; /* + vmcoreinfo */ nr_phdr +=3D mem->nr_ranges; nr_phdr++; /* + kernel text map */ So the reservation covers nr_ranges + 2 + CONFIG_NR_CPUS_DEFAULT phdrs while the buffer holds nr_ranges + 2 + num_possible_cpus() phdrs, and the buffer exceeds the reservation by (num_possible_cpus() - CONFIG_NR_CPUS_DEFAULT) * sizeof(Elf64_Phdr) bytes as soon as num_possible_cpus() grows past CONFIG_NR_CPUS_DEFAULT. num_possible_cpus() is bounded by CONFIG_NR_CPUS, not by CONFIG_NR_CPUS_DEFAULT, so this is reachable on any config that raises CONFIG_NR_CPUS above the arch default without CONFIG_MAXSMP. crash_prepare_elf64_headers() rounds bufsz up to ELF_CORE_HEADER_ALIGN and kexec_add_buffer() rounds memsz up to PAGE_SIZE (both 4096), so the excess is invisible until it outgrows that padding. Once it does, sanity_check_segment_list() rejects the image: if (image->segment[i].bufsz > image->segment[i].memsz) return -EINVAL; kexec_load(2) does not report an error on the same machine, but it is not unaffected either. With crash hotplug enabled, kexec-tools sizes the elfcorehdr segment from /sys/kernel/crash_elfcorehdr_size in load_crashdump_segments(), i.e. from arch_crash_get_elfcorehdr_size(), which derives its answer from the very same CONFIG_NR_CPUS_DEFAULT. add_segment_phys_virt() then clamps the buffer to that segment: if (bufsz > memsz) { bufsz =3D memsz; } so on the kexec_load(2) path the undersized reservation silently truncates the elfcorehdr instead of failing the load, which surfaces later as a bad or unusable dump rather than as a load failure. Switching arch_crash_get_elfcorehdr_size() to CONFIG_NR_CPUS below fixes that path as well. Observed on a single-socket Xeon 6776P (144 possible CPUs) running a PREEMPT_RT kernel with: # CONFIG_MAXSMP is not set # CONFIG_MEMORY_HOTPLUG is not set CONFIG_NR_CPUS_RANGE_BEGIN=3D2 CONFIG_NR_CPUS_RANGE_END=3D512 CONFIG_NR_CPUS_DEFAULT=3D64 CONFIG_NR_CPUS=3D256 At 144 possible CPUs the buffer exceeds the reservation by (144 - 64) * 56 =3D 4480 bytes. That is more than the 4096 bytes of page padding, so the overflow is guaranteed and kexec -p -s fails with "kexec_file_load failed: Invalid argument". Reducing the possible CPU count to 72 leaves an excess of (72 - 64) * 56 =3D 448 bytes, which the page rounding still absorbs, and the load succeeds - confirming the reservation is the limiting factor. Reserve the elfcorehdr for CONFIG_NR_CPUS, the compile-time upper bound of num_possible_cpus(), so the reservation always covers the header that is actually generated. The CONFIG_MEMORY_HOTPLUG=3Dy path discards @pnum and reserves 2 + CONFIG_NR_CPUS_DEFAULT + CONFIG_CRASH_MAX_MEMORY_RANGES phdrs instead. With the default CONFIG_CRASH_MAX_MEMORY_RANGES=3D8192 the memory range allowance dwarfs the CPU shortfall, so that path does not fail in practice; it is switched to CONFIG_NR_CPUS as well for consistency and to stay correct for small CONFIG_CRASH_MAX_MEMORY_RANGES values. This does not change the reservation for defconfig-like builds, since CONFIG_NR_CPUS defaults to CONFIG_NR_CPUS_DEFAULT. Only configs that raise CONFIG_NR_CPUS reserve more, and the worst case is bounded by the top of the range (CONFIG_NR_CPUS=3D8192 with CONFIG_CPUMASK_OFFSTACK=3Dy), which is exactly what CONFIG_MAXSMP already reserves today. Fixes: ea53ad9cf73b ("x86/crash: add x86 crash hotplug support") Assisted-by: LLM Signed-off-by: Ionut Nechita Reviewed-by: Jinjie Ruan Reviewed-by: Bradley Morgan Reviewed-by: Sourabh Jain Acked-by: Baoquan He --- arch/x86/kernel/crash.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c index e681ec9cf1dc..e6f23933a6df 100644 --- a/arch/x86/kernel/crash.c +++ b/arch/x86/kernel/crash.c @@ -369,9 +369,9 @@ int crash_load_segments(struct kimage *image) * maximum CPUs and maximum memory ranges. */ if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) - pnum =3D 2 + CONFIG_NR_CPUS_DEFAULT + CONFIG_CRASH_MAX_MEMORY_RANGES; + pnum =3D 2 + CONFIG_NR_CPUS + CONFIG_CRASH_MAX_MEMORY_RANGES; else - pnum +=3D 2 + CONFIG_NR_CPUS_DEFAULT; + pnum +=3D 2 + CONFIG_NR_CPUS; =20 if (pnum < (unsigned long)PN_XNUM) { kbuf.memsz =3D pnum * sizeof(Elf64_Phdr); @@ -430,7 +430,7 @@ unsigned int arch_crash_get_elfcorehdr_size(void) unsigned int sz; =20 /* 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Wed, 26 Aug 2026 07:35:53 +0000 From: "Ionut Nechita (Wind River)" To: x86@kernel.org, kexec@lists.infradead.org Cc: tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, akpm@linux-foundation.org, baoquan.he@linux.dev, rppt@kernel.org, pasha.tatashin@soleen.com, pratyush@kernel.org, ruirui.yang@linux.dev, eric.devolder@oracle.com, hbathini@linux.ibm.com, sourabhjain@linux.ibm.com, ruanjinjie@huawei.com, include@grrlz.net, linux-kernel@vger.kernel.org Subject: [PATCH v3 2/2] crash: update stale NR_CPUS_DEFAULT references in elfcorehdr sizing docs Date: Wed, 26 Aug 2026 10:35:27 +0300 Message-ID: <20260826073527.21487-3-ionut.nechita@windriver.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260826073527.21487-1-ionut.nechita@windriver.com> References: <20260826073527.21487-1-ionut.nechita@windriver.com> Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: VI1PR0102CA0108.eurprd01.prod.exchangelabs.com (2603:10a6:803:15::49) To 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elfcorehdr over-allocation for crash hotplug is now computed from CONFIG_NR_CPUS rather than CONFIG_NR_CPUS_DEFAULT, but two pieces of documentation still name the old symbol: the comment above crash_handle_hotplug_event() and the CRASH_MAX_MEMORY_RANGES help text. Update both so they describe what the code actually does and do not mislead people sizing CRASH_MAX_MEMORY_RANGES. Documentation only, no functional change. Suggested-by: Jinjie Ruan Suggested-by: Bradley Morgan Assisted-by: LLM Signed-off-by: Ionut Nechita Reviewed-by: Bradley Morgan --- kernel/Kconfig.kexec | 2 +- kernel/crash_core.c | 2 +- 2 files changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/Kconfig.kexec b/kernel/Kconfig.kexec index 15632358bcf7..a97ed9605602 100644 --- a/kernel/Kconfig.kexec +++ b/kernel/Kconfig.kexec @@ -167,7 +167,7 @@ config CRASH_MAX_MEMORY_RANGES memory regions that the elfcorehdr buffer/segment can accommodate. These regions are obtained via walk_system_ram_res(); eg. the 'System RAM' entries in /proc/iomem. - This value is combined with NR_CPUS_DEFAULT and multiplied by + This value is combined with NR_CPUS and multiplied by sizeof(Elf64_Phdr) to determine the final elfcorehdr memory buffer/ segment size. The value 8192, for example, covers a (sparsely populated) 1TiB system diff --git a/kernel/crash_core.c b/kernel/crash_core.c index 2b36aa9fade0..d0bd2d0cf899 100644 --- a/kernel/crash_core.c +++ b/kernel/crash_core.c @@ -648,7 +648,7 @@ int crash_check_hotplug_support(void) * new list of CPUs and memory. To make changes to the elfcorehdr, it * should be large enough to permit a growing number of CPU and Memory * resources. One can estimate the elfcorehdr memory size based on - * NR_CPUS_DEFAULT and CRASH_MAX_MEMORY_RANGES. The elfcorehdr is + * NR_CPUS and CRASH_MAX_MEMORY_RANGES. The elfcorehdr is * excluded from SHA verification by default if the architecture * supports crash hotplug. */ --=20 2.55.0