Re: [RFC][PATCH 1/3] powerpc: Don't force ENOSYS as error on syscall fail

On 11.02.2015 05:04, Michael Ellerman wrote:

On Mon, 2015-02-09 at 07:55 +0000, Bogdan Purcareata wrote:

quoted

In certain scenarios - e.g. seccomp filtering with ERRNO as default action -
the system call fails for other reasons than the syscall not being available.
The seccomp filter can be configured to store a user-defined error code on
return from a blacklisted syscall.

The RFC is this: are there currently any user-space scenarios where it is
required that the system call return ENOSYS as error code on failure, no matter
the circumstances? I don't want to break userspace requirements. I have not
added code to force this error code in situations different than
secure_computing failure, in order to keep overhead at a minimum.

Signed-off-by: Bogdan Purcareata <redacted>
---
  arch/powerpc/kernel/entry_32.S | 3 ++-
  arch/powerpc/kernel/entry_64.S | 2 +-
  2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index 59848e5..52e48dd 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S

@@ -425,7 +425,8 @@ END_FTR_SECTION_IFSET(CPU_FTR_NEED_PAIRED_STWCX)
  	b	1b
  #endif  /* CONFIG_44x */

-66:	li	r3,-ENOSYS
+66:
+#	li	r3,-ENOSYS
  	b	ret_from_syscall

  	.globl	ret_from_fork

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index e6bfe8e..80db02e 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S

@@ -269,7 +269,7 @@ syscall_dotrace:
  	b	.Lsyscall_dotrace_cont

  syscall_enosys:
-	li	r3,-ENOSYS
+#	li	r3,-ENOSYS
  	b	syscall_exit

So what happens if you call this with a syscall number that's out of bounds?

As far as my current understanding goes, the call will return with -1 
with a errno that's undefined (or I've not seen it be defined anywhere).

I've thought more about this, and I guess the best option would be to 
move setting -ENOSYS as errno from the syscall entry assembly to 
do_syscall_trace_enter (as opposed to eliminating it at all). I was a 
little reluctant to do this at first in order to keep overhead to a 
minimum, but it's certainly not an option to change behavior if the 
syscall number is out of bounds.

v2 to come shortly.

Thanks,
Bogdan P.