[PATCH v5 1/6] x86/elf: Add table to document READ_IMPLIES_EXEC

[Kees Cook]

Add a table to document the current behavior of READ_IMPLIES_EXEC in
preparation for changing the behavior.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
---
 arch/x86/include/asm/elf.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 69c0f892e310..ee459d4c3b45 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -281,6 +281,25 @@ extern u32 elf_hwcap2;
 /*
  * An executable for which elf_read_implies_exec() returns TRUE will
  * have the READ_IMPLIES_EXEC personality flag set automatically.
+ *
+ * The decision process for determining the results are:
+ *
+ * Â Â Â Â Â Â    ÂCPU: | lacks NX* Â| has NX, ia32 Â Â | has NX, x86_64 |
+ * ELF: Â Â Â Â    Â Â Â| Â Â Â Â Â Â| Â Â Â Â Â Â Â Â Â| Â Â Â Â Â Â Â Â|
+ * ---------------------|------------|------------------|----------------|
+ * missing PT_GNU_STACK | exec-all  | exec-all     | exec-all    |
+ * PT_GNU_STACK == RWX Â| exec-all  | exec-all     | exec-all    |
+ * PT_GNU_STACK == RW  | exec-none Â| exec-none    Â| exec-none   Â|
+ *
+ *  exec-all  : all PROT_READ user mappings are executable, except when
+ *              backed by files on a noexec-filesystem.
+ *  exec-none : only PROT_EXEC user mappings are executable.
+ *
+ *  *this column has no architectural effect: NX markings are ignored by
+ *   hardware, but may have behavioral effects when "wants X" collides with
+ *   "cannot be X" constraints in memory permission flags, as in
+ *   https://lkml.kernel.org/r/20190418055759.GA3155@xxxxxxxxxxxx
+ *
  */
 #define elf_read_implies_exec(ex, executable_stack)	\
 	(executable_stack != EXSTACK_DISABLE_X)
-- 
2.20.1