Re: [PATCH v4 1/2] module: add elf_check_module_arch for module specific elf arch checks

+++ Nicholas Piggin [15/06/21 12:05 +1000]:

Excerpts from Jessica Yu's message of June 14, 2021 10:06 pm:

quoted

+++ Nicholas Piggin [11/06/21 19:39 +1000]:

quoted

The elf_check_arch() function is used to test usermode binaries, but
kernel modules may have more specific requirements. powerpc would like
to test for ABI version compatibility.

Add an arch-overridable function elf_check_module_arch() that defaults
to elf_check_arch() and use it in elf_validity_check().

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
[np: split patch, added changelog]
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
include/linux/moduleloader.h | 5 +++++
kernel/module.c              | 2 +-
2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/moduleloader.h b/include/linux/moduleloader.h
index 9e09d11ffe5b..fdc042a84562 100644
--- a/include/linux/moduleloader.h
+++ b/include/linux/moduleloader.h

@@ -13,6 +13,11 @@
 * must be implemented by each architecture.
 */

+// Allow arch to optionally do additional checking of module ELF header
+#ifndef elf_check_module_arch
+#define elf_check_module_arch elf_check_arch
+#endif

Hi Nicholas,

Why not make elf_check_module_arch() consistent with the other
arch-specific functions? Please see module_frob_arch_sections(),
module_{init,exit}_section(), etc in moduleloader.h. That is, they are
all __weak functions that are overridable by arches. We can maybe make
elf_check_module_arch() a weak symbol, available for arches to
override if they want to perform additional elf checks. Then we don't
have to have this one-off #define.

Like this? I like it. Good idea.

Yeah! Also, maybe we can alternatively make elf_check_module_arch() a
separate check entirely so that the powerpc implementation doesn't
have to include that extra elf_check_arch() call. Something like this maybe?

diff --git a/include/linux/moduleloader.h b/include/linux/moduleloader.h
index 9e09d11ffe5b..2f9ebd593b4f 100644
--- a/include/linux/moduleloader.h
+++ b/include/linux/moduleloader.h

@@ -39,6 +39,9 @@ bool module_init_section(const char *name);
  */
 bool module_exit_section(const char *name);

+/* Arch may override to do additional checking of ELF header architecture */
+int elf_check_module_arch(Elf_Ehdr *hdr);
+
 /*
  * Apply the given relocation to the (simplified) ELF.  Return -error
  * or 0.

diff --git a/kernel/module.c b/kernel/module.c
index fdd6047728df..9963a979ed54 100644
--- a/kernel/module.c
+++ b/kernel/module.c

@@ -2923,6 +2923,11 @@ static int validate_section_offset(struct load_info *info, Elf_Shdr *shdr)
        return 0;
 }

+int __weak elf_check_module_arch(Elf_Ehdr *hdr)
+{
+       return 1;
+}
+
 /*
  * Sanity checks against invalid binaries, wrong arch, weird elf version.
  *

@@ -2941,6 +2946,7 @@ static int elf_validity_check(struct load_info *info)
        if (memcmp(info->hdr->e_ident, ELFMAG, SELFMAG) != 0
            || info->hdr->e_type != ET_REL
            || !elf_check_arch(info->hdr)
+           || !elf_check_module_arch(info->hdr)
            || info->hdr->e_shentsize != sizeof(Elf_Shdr))
                return -ENOEXEC;