binfmt_elf: preserve original ELF e_flags for core dumps

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@syntacore.com>
Link: https://lore.kernel.org/r/20250901135350.619485-1-svetlana.parfenova@syntacore.com
Signed-off-by: Kees Cook <kees@kernel.org>

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index a4b233a..b06a758 100644 (file)
--- 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 bd2f530..1949e25 100644 (file)
--- 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 4aacf9c..e4653bb 100644 (file)
--- 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 @@ out_free_interp:
        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 08bc244..04a2857 100644 (file)
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1102,6 +1102,11 @@ 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;