[PATCH] FDPIC: Ignore the loader's PT_GNU_STACK when calculating the stack size

[David Howells <dhowells@redhat.com>]

Ignore the loader's PT_GNU_STACK when calculating the stack size, and only
consider the executable's PT_GNU_STACK, assuming the executable has one.

Currently the behaviour is to take the largest stack size and use that, but
that means you can't reduce the stack size in the executable.  The loader's
stack size should probably only be used when executing the loader directly.

WARNING: This patch is slightly dangerous - it may render a system inoperable
if the loader's stack size is larger than that of important executables, and
the system relies unknowingly on this increasing the size of the stack.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/binfmt_elf_fdpic.c |   19 +++++++++++--------
 1 files changed, 11 insertions(+), 8 deletions(-)


diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
index 81ca047..58c49fa 100644
--- a/fs/binfmt_elf_fdpic.c
+++ b/fs/binfmt_elf_fdpic.c
@@ -283,20 +283,23 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm,
 	}
 
 	stack_size = exec_params.stack_size;
-	if (stack_size < interp_params.stack_size)
-		stack_size = interp_params.stack_size;
-
 	if (exec_params.flags & ELF_FDPIC_FLAG_EXEC_STACK)
 		executable_stack = EXSTACK_ENABLE_X;
 	else if (exec_params.flags & ELF_FDPIC_FLAG_NOEXEC_STACK)
 		executable_stack = EXSTACK_DISABLE_X;
-	else if (interp_params.flags & ELF_FDPIC_FLAG_EXEC_STACK)
-		executable_stack = EXSTACK_ENABLE_X;
-	else if (interp_params.flags & ELF_FDPIC_FLAG_NOEXEC_STACK)
-		executable_stack = EXSTACK_DISABLE_X;
-	else
+	else 
 		executable_stack = EXSTACK_DEFAULT;
 
+	if (stack_size == 0) {
+		stack_size = interp_params.stack_size;
+		if (interp_params.flags & ELF_FDPIC_FLAG_EXEC_STACK)
+			executable_stack = EXSTACK_ENABLE_X;
+		else if (interp_params.flags & ELF_FDPIC_FLAG_NOEXEC_STACK)
+			executable_stack = EXSTACK_DISABLE_X;
+		else
+			executable_stack = EXSTACK_DEFAULT;
+	}
+
 	retval = -ENOEXEC;
 	if (stack_size == 0)
 		goto error;