[PATCH v5 15/17] crash: Introduce crash_extra_elfcorehdr_size() helper

[Jinjie Ruan]

Extract the elfcorehdr extra space calculation from powerpc into a
generic helper crash_extra_elfcorehdr_size() for use by other
architectures like arm64.

Strengthen the original powerpc check: instead of only checking
the loose CONFIG_CRASH_MAX_MEMORY_RANGES, the new helper enforces
a strict compile-time BUILD_BUG_ON() to guarantee that the absolute
maximum theoretical number of ELF Program Headers will never exceed
the ELF physical limit of PN_XNUM. This ensures absolute safety across
all architectures with zero runtime overhead.

The helper also provides a zero-size stub when crash memory hotplug
is disabled.

Cc: Madhavan Srinivasan <maddy at linux.ibm.com>
Cc: Michael Ellerman <mpe at ellerman.id.au>
Cc: Nicholas Piggin <npiggin at gmail.com>
Cc: "Christophe Leroy (CS GROUP)" <chleroy at kernel.org>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Baoquan He <baoquan.he at linux.dev>
Cc: Mike Rapoport <rppt at kernel.org>
Cc: Pasha Tatashin <pasha.tatashin at soleen.com>
Cc: Pratyush Yadav <pratyush at kernel.org>
Cc: Dave Young <ruirui.yang at linux.dev>
Cc: Sourabh Jain <sourabhjain at linux.ibm.com>
Signed-off-by: Jinjie Ruan <ruanjinjie at huawei.com>
---
 arch/powerpc/kexec/file_load_64.c | 15 +--------------
 include/linux/crash_core.h        | 20 ++++++++++++++++++++
 2 files changed, 21 insertions(+), 14 deletions(-)

diff --git a/arch/powerpc/kexec/file_load_64.c b/arch/powerpc/kexec/file_load_64.c
index bd80c5fb1b1f..d3512c85ea83 100644
--- a/arch/powerpc/kexec/file_load_64.c
+++ b/arch/powerpc/kexec/file_load_64.c
@@ -374,19 +374,6 @@ static int load_backup_segment(struct kimage *image, struct kexec_buf *kbuf)
 	return 0;
 }
 
-static unsigned int kdump_extra_elfcorehdr_size(struct crash_mem *cmem)
-{
-#if defined(CONFIG_CRASH_HOTPLUG) && defined(CONFIG_MEMORY_HOTPLUG)
-	if (CONFIG_CRASH_MAX_MEMORY_RANGES > (unsigned int)PN_XNUM)
-		pr_warn("Number of Phdrs %u exceeds max\n", CONFIG_CRASH_MAX_MEMORY_RANGES);
-	else if (cmem->nr_ranges >= CONFIG_CRASH_MAX_MEMORY_RANGES)
-		pr_warn("Configured crash mem ranges may not be enough\n");
-	else
-		return (CONFIG_CRASH_MAX_MEMORY_RANGES - cmem->nr_ranges) * sizeof(Elf64_Phdr);
-#endif
-	return 0;
-}
-
 /**
  * load_elfcorehdr_segment - Setup crash memory ranges and initialize elfcorehdr
  *                           segment needed to load kdump kernel.
@@ -424,7 +411,7 @@ static int load_elfcorehdr_segment(struct kimage *image, struct kexec_buf *kbuf)
 	 * Account for extra space required to accommodate additional memory
 	 * ranges in elfcorehdr due to memory hotplug events.
 	 */
-	kbuf->memsz = headers_sz + kdump_extra_elfcorehdr_size(cmem);
+	kbuf->memsz = headers_sz + crash_extra_elfcorehdr_size(cmem->nr_ranges);
 	kbuf->top_down = false;
 
 	ret = kexec_add_buffer(kbuf);
diff --git a/include/linux/crash_core.h b/include/linux/crash_core.h
index 1296a9b29974..aee2449f343b 100644
--- a/include/linux/crash_core.h
+++ b/include/linux/crash_core.h
@@ -107,4 +107,24 @@ ssize_t dm_crypt_keys_read(char *buf, size_t count, u64 *ppos);
 static inline int crash_load_dm_crypt_keys(struct kimage *image) {return 0; }
 #endif
 
+#if defined(CONFIG_CRASH_HOTPLUG) && defined(CONFIG_MEMORY_HOTPLUG)
+static inline unsigned int crash_extra_elfcorehdr_size(unsigned int nr_mem_ranges)
+{
+	BUILD_BUG_ON((2 + CONFIG_NR_CPUS + CONFIG_CRASH_MAX_MEMORY_RANGES) >=
+		     (unsigned int)PN_XNUM);
+
+	if (nr_mem_ranges >= CONFIG_CRASH_MAX_MEMORY_RANGES) {
+		pr_warn_once("Configured crash mem ranges may not be enough\n");
+		return 0;
+	}
+
+	return (CONFIG_CRASH_MAX_MEMORY_RANGES - nr_mem_ranges) * sizeof(Elf64_Phdr);
+}
+#else
+static inline unsigned int crash_extra_elfcorehdr_size(unsigned int nr_mem_ranges)
+{
+	return 0;
+}
+#endif
+
 #endif /* LINUX_CRASH_CORE_H */
-- 
2.34.1