[RFC] binfmt_elf: preserve original ELF e_flags in core dumps

[Svetlana Parfenova <svetlana.parfenova@syntacore.com>]

Preserve the original ELF e_flags from the executable in the core dump
header instead of relying on compile-time defaults (ELF_CORE_EFLAGS or
value from the regset view). This ensures that ABI-specific flags in
the dump file match the actual binary being executed.

Save the e_flags field during ELF binary loading (in load_elf_binary())
into the mm_struct, and later retrieve it during core dump generation
(in fill_note_info()). Use this saved value to populate the e_flags in
the core dump ELF header.

Add a new Kconfig option, CONFIG_CORE_DUMP_USE_PROCESS_EFLAGS, to guard
this behavior. Although motivated by a RISC-V use case, the mechanism is
generic and can be applied to all architectures.

This change is needed to resolve a debugging issue encountered when
analyzing core dumps with GDB for RISC-V systems. GDB inspects the
e_flags field to determine whether optional register sets such as the
floating-point unit are supported. Without correct flags, GDB may warn
and ignore valid register data:

    warning: Unexpected size of section '.reg2/213' in core file.

As a result, floating-point registers are not accessible in the debugger,
even though they were dumped. Preserving the original e_flags enables
GDB and other tools to properly interpret the dump contents.

Signed-off-by: Svetlana Parfenova <svetlana.parfenova@syntacore.com>
---
 fs/Kconfig.binfmt        |  9 +++++++++
 fs/binfmt_elf.c          | 26 ++++++++++++++++++++------
 include/linux/mm_types.h |  5 +++++
 3 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/fs/Kconfig.binfmt b/fs/Kconfig.binfmt
index bd2f530e5740..45bed2041542 100644
--- a/fs/Kconfig.binfmt
+++ b/fs/Kconfig.binfmt
@@ -184,4 +184,13 @@ config EXEC_KUNIT_TEST
 	  This builds the exec KUnit tests, which tests boundary conditions
 	  of various aspects of the exec internals.
 
+config CORE_DUMP_USE_PROCESS_EFLAGS
+	bool "Preserve ELF e_flags from executable in core dumps"
+	depends on BINFMT_ELF && ELF_CORE && RISCV
+	default n
+	help
+	  Save the ELF e_flags from the process executable at load time
+	  and use it in the core dump header. This ensures the dump reflects
+	  the original binary ABI.
+
 endmenu
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 4c1ea6b52a53..baf749e431a1 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1297,6 +1297,11 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	mm->end_data = end_data;
 	mm->start_stack = bprm->p;
 
+#ifdef CONFIG_CORE_DUMP_USE_PROCESS_EFLAGS
+	/* stash e_flags for use in core dumps */
+	mm->saved_e_flags = elf_ex->e_flags;
+#endif
+
 	/**
 	 * DOC: "brk" handling
 	 *
@@ -1870,6 +1875,8 @@ static int fill_note_info(struct elfhdr *elf, int phdrs,
 	struct elf_thread_core_info *t;
 	struct elf_prpsinfo *psinfo;
 	struct core_thread *ct;
+	u16 machine;
+	u32 flags;
 
 	psinfo = kmalloc(sizeof(*psinfo), GFP_KERNEL);
 	if (!psinfo)
@@ -1897,17 +1904,24 @@ static int fill_note_info(struct elfhdr *elf, int phdrs,
 		return 0;
 	}
 
-	/*
-	 * Initialize the ELF file header.
-	 */
-	fill_elf_header(elf, phdrs,
-			view->e_machine, view->e_flags);
+	machine = view->e_machine;
+	flags = view->e_flags;
 #else
 	view = NULL;
 	info->thread_notes = 2;
-	fill_elf_header(elf, phdrs, ELF_ARCH, ELF_CORE_EFLAGS);
+	machine = ELF_ARCH;
+	flags = ELF_CORE_EFLAGS;
 #endif
 
+#ifdef CONFIG_CORE_DUMP_USE_PROCESS_EFLAGS
+	flags = dump_task->mm->saved_e_flags;
+#endif
+
+	/*
+	 * Initialize the ELF file header.
+	 */
+	fill_elf_header(elf, phdrs, machine, flags);
+
 	/*
 	 * Allocate a structure for each thread.
 	 */
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 56d07edd01f9..5487d6ba6fcb 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1059,6 +1059,11 @@ struct mm_struct {
 
 		unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */
 
+#ifdef CONFIG_CORE_DUMP_USE_PROCESS_EFLAGS
+		/* the ABI-related flags from the ELF header. Used for core dump */
+		unsigned long saved_e_flags;
+#endif
+
 		struct percpu_counter rss_stat[NR_MM_COUNTERS];
 
 		struct linux_binfmt *binfmt;
-- 
2.39.5