[PATCH AUTOSEL 6.17-6.12] binfmt_elf: preserve original ELF e_flags for core dumps
Kees Cook
kees at kernel.org
Thu Oct 2 08:58:30 PDT 2025
I'm not excited about this getting backported without an explicit
user/champion for it -- it adds Kconfig dependencies and potentially
changes existing runtime behaviors.
-Kees
On Thu, Oct 02, 2025 at 11:30:18AM -0400, Sasha Levin wrote:
> From: Svetlana Parfenova <svetlana.parfenova at syntacore.com>
>
> [ Upstream commit 8c94db0ae97c72c253a615f990bd466b456e94f6 ]
>
> Some architectures, such as RISC-V, use the ELF e_flags field to encode
> ABI-specific information (e.g., ISA extensions, fpu support). Debuggers
> like GDB rely on these flags in core dumps to correctly interpret
> optional register sets. If the flags are missing or incorrect, GDB may
> warn and ignore valid data, for example:
>
> warning: Unexpected size of section '.reg2/213' in core file.
>
> This can prevent access to fpu or other architecture-specific registers
> even when they were dumped.
>
> 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()). Kconfig option CONFIG_ARCH_HAS_ELF_CORE_EFLAGS
> is introduced for architectures that require this behaviour.
>
> Signed-off-by: Svetlana Parfenova <svetlana.parfenova at syntacore.com>
> Link: https://lore.kernel.org/r/20250901135350.619485-1-svetlana.parfenova@syntacore.com
> Signed-off-by: Kees Cook <kees at kernel.org>
> Signed-off-by: Sasha Levin <sashal at kernel.org>
> ---
>
> LLM Generated explanations, may be completely bogus:
>
> Based on my comprehensive analysis, here is my determination:
>
> ---
>
> ## **Backport Status: YES**
>
> ### **Extensive Analysis:**
>
> #### **Problem Being Fixed:**
> This commit addresses a debugging issue on RISC-V architecture where GDB
> cannot properly interpret core dumps. The root cause is that RISC-V uses
> the ELF e_flags field to encode critical ABI information (ISA
> extensions, FPU support), and without preserving these flags in core
> dumps, GDB displays warnings like:
>
> ```
> warning: Unexpected size of section '.reg2/213' in core file.
> ```
>
> This prevents developers from accessing FPU or other architecture-
> specific registers during post-mortem debugging, even when the data was
> correctly dumped.
>
> #### **Code Changes Analysis:**
>
> **1. fs/binfmt_elf.c (core changes):**
> - Lines 106-119: Adds two inline helper functions
> `elf_coredump_set_mm_eflags()` and `elf_coredump_get_mm_eflags()`
> that are no-ops when CONFIG_ARCH_HAS_ELF_CORE_EFLAGS is disabled
> - Line 1308: Saves e_flags during ELF binary loading:
> `elf_coredump_set_mm_eflags(mm, elf_ex->e_flags)`
> - Lines 1824-1866: Refactors fill_note_info() to extract machine type
> and flags into local variables, then overrides flags with saved
> value if architecture opted in
>
> **2. include/linux/mm_types.h:**
> - Adds `unsigned long saved_e_flags` field to mm_struct,
> conditionally compiled
> - Field placement is logical (after saved_auxv, another ELF-related
> field)
> - Memory overhead: one unsigned long (8 bytes on 64-bit) per
> mm_struct, only on RISC-V
>
> **3. fs/Kconfig.binfmt:**
> - Adds CONFIG_ARCH_HAS_ELF_CORE_EFLAGS option
> - Depends on BINFMT_ELF && ELF_CORE
> - Allows architectures to opt-in to e_flags preservation
>
> **4. arch/riscv/Kconfig:**
> - Enables CONFIG_ARCH_HAS_ELF_CORE_EFLAGS for RISC-V
>
> #### **Why This Qualifies for Backporting:**
>
> **✓ Important Bug Fix:**
> - Fixes real user-facing problem: debugging RISC-V applications with FPU
> is broken
> - Affects developers working on RISC-V platforms
> - No workaround available (the information is lost in core dumps)
>
> **✓ Small and Contained:**
> - Net change: 55 insertions, 6 deletions across 4 files
> - All logic is simple save/restore pattern
> - No complex state management or synchronization
>
> **✓ Minimal Regression Risk:**
> - Opt-in via Kconfig - other architectures completely unaffected
> - When disabled: zero runtime overhead (functions compile to empty
> inline stubs)
> - When enabled: trivial assignment operations with no failure paths
> - Changes are in well-established code paths (load_elf_binary,
> fill_note_info)
>
> **✓ No Dependencies:**
> - Applies cleanly to 6.17 tree
> - No prerequisite commits required
> - The backport also includes a728ce8ffbd27 (struct_size() refactoring)
> which is a compatible cleanup
>
> **✓ Stable After Mainline:**
> - Committed Sep 3, 2025 (27 days in mainline as of Sep 30)
> - No fixes, reverts, or reported issues
> - git log shows no follow-up corrections needed
>
> **✓ Maintainer Approved:**
> - Signed-off-by: Kees Cook (execve/binfmt maintainer)
> - Proper review process through lore.kernel.org
>
> **✓ Follows Stable Kernel Rules:**
> - Rule 1: Must be obviously correct and tested ✓
> - Rule 2: Significant bugfix ✓
> - Rule 3: Not larger than 100 lines ✓
> - Rule 4: Patches cannot add features ✓ (fixes existing feature)
> - Rule 5: No theoretical race condition fixes ✓ (fixes actual GDB issue)
>
> #### **Technical Correctness:**
>
> **ELF_CORE_EFLAGS Historical Context:**
> - The macro has existed since 2005 (Linus's original git import)
> - Only Hexagon and PowerPC defined non-zero values
> - RISC-V's e_flags usage is architecturally critical for ABI
> compatibility
> - The old approach (using static ELF_CORE_EFLAGS=0) lost runtime
> information
>
> **Design Quality:**
> - Clean separation: CONFIG option → mm_struct field → helper functions
> - Backward compatible: architectures that don't need it pay zero cost
> - Forward compatible: other architectures (ARM, MIPS) could adopt this
> if needed
> - Proper abstraction through inline helpers prevents #ifdef
> proliferation
>
> #### **Potential Concerns Addressed:**
>
> **Q: Is mm_struct size increase acceptable?**
> A: Yes. One unsigned long (8 bytes) per process on RISC-V only,
> negligible compared to typical mm_struct size (~1KB+)
>
> **Q: Why not use regset_view->e_flags?**
> A: The regset_view structure doesn't have an e_flags field. The old code
> fell back to the compile-time ELF_CORE_EFLAGS constant, losing the per-
> process runtime ABI information from the actual ELF binary.
>
> **Q: Thread safety concerns?**
> A: None. The e_flags are written once during load_elf_binary() before
> any threads can be created, and read during core dump when all threads
> are stopped.
>
> **Q: Is this too recent for backporting?**
> A: While 27 days is relatively fresh, the change is so minimal and
> isolated that the risk is negligible. The opt-in nature means
> regressions can only affect RISC-V users who explicitly enable
> coredumps.
>
> ### **Conclusion:**
>
> This commit should be backported because it fixes an important debugging
> issue on RISC-V with minimal risk. The change is small, self-contained,
> opt-in, and has been stable in mainline. It meets all stable kernel
> backporting criteria and will significantly improve the debugging
> experience for RISC-V developers without impacting other architectures.
>
> arch/riscv/Kconfig | 1 +
> fs/Kconfig.binfmt | 9 +++++++++
> fs/binfmt_elf.c | 40 ++++++++++++++++++++++++++++++++++------
> include/linux/mm_types.h | 5 +++++
> 4 files changed, 49 insertions(+), 6 deletions(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 51dcd8eaa2435..74db054aa1b8b 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -28,6 +28,7 @@ config RISCV
> select ARCH_HAS_DEBUG_VIRTUAL if MMU
> select ARCH_HAS_DEBUG_VM_PGTABLE
> select ARCH_HAS_DEBUG_WX
> + select ARCH_HAS_ELF_CORE_EFLAGS
> select ARCH_HAS_FAST_MULTIPLIER
> select ARCH_HAS_FORTIFY_SOURCE
> select ARCH_HAS_GCOV_PROFILE_ALL
> diff --git a/fs/Kconfig.binfmt b/fs/Kconfig.binfmt
> index bd2f530e57408..1949e25c7741b 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 ARCH_HAS_ELF_CORE_EFLAGS
> + bool
> + depends on BINFMT_ELF && ELF_CORE
> + default n
> + help
> + Select this option if the architecture makes use of the e_flags
> + field in the ELF header to store ABI or other architecture-specific
> + information that should be preserved in core dumps.
> +
> endmenu
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 264fba0d44bdf..c126e3d0e7018 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -103,6 +103,21 @@ static struct linux_binfmt elf_format = {
>
> #define BAD_ADDR(x) (unlikely((unsigned long)(x) >= TASK_SIZE))
>
> +static inline void elf_coredump_set_mm_eflags(struct mm_struct *mm, u32 flags)
> +{
> +#ifdef CONFIG_ARCH_HAS_ELF_CORE_EFLAGS
> + mm->saved_e_flags = flags;
> +#endif
> +}
> +
> +static inline u32 elf_coredump_get_mm_eflags(struct mm_struct *mm, u32 flags)
> +{
> +#ifdef CONFIG_ARCH_HAS_ELF_CORE_EFLAGS
> + flags = mm->saved_e_flags;
> +#endif
> + return flags;
> +}
> +
> /*
> * We need to explicitly zero any trailing portion of the page that follows
> * p_filesz when it ends before the page ends (e.g. bss), otherwise this
> @@ -1290,6 +1305,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
> mm->end_data = end_data;
> mm->start_stack = bprm->p;
>
> + elf_coredump_set_mm_eflags(mm, elf_ex->e_flags);
> +
> /**
> * DOC: "brk" handling
> *
> @@ -1804,6 +1821,8 @@ 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)
> @@ -1831,17 +1850,26 @@ 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
>
> + /*
> + * Override ELF e_flags with value taken from process,
> + * if arch needs that.
> + */
> + flags = elf_coredump_get_mm_eflags(dump_task->mm, flags);
> +
> + /*
> + * 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 a643fae8a3494..7f625c35128be 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1107,6 +1107,11 @@ struct mm_struct {
>
> unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */
>
> +#ifdef CONFIG_ARCH_HAS_ELF_CORE_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;
> --
> 2.51.0
>
--
Kees Cook
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