Re: [PATCH RFC 1/1] Documentation: describe how to add a system call

On Tue, Jul 28, 2015 at 5:43 PM, Kees Cook [off-list ref] wrote:

On Tue, Jul 28, 2015 at 4:41 AM, David Drysdale [off-list ref] wrote:

quoted

Add a document describing the process of adding a new system call,
including the need for a flags argument for future compatibility, and
covering 32-bit/64-bit concerns (albeit in an x86-centric way).

Signed-off-by: David Drysdale <redacted>
Reviewed-by: Michael Kerrisk <redacted>

This is great!

Reviewed-by: Kees Cook <redacted>

I have a few minor suggestions below...

Thanks, I've applied all bar one -- a query below.

quoted

---
 Documentation/adding-syscalls.txt | 454 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 454 insertions(+)
 create mode 100644 Documentation/adding-syscalls.txt

diff --git a/Documentation/adding-syscalls.txt b/Documentation/adding-syscalls.txt
new file mode 100644
index 000000000000..5f52edda8951
--- /dev/null
+++ b/Documentation/adding-syscalls.txt

[snip]

quoted

+ - If there is an existing capability that governs related functionality, then
+   use that.  However, avoid combining lots of only vaguely related functions
+   together under the same bit, as this goes against capabilities' purpose of
+   splitting the power of root.  In particular, avoid adding new uses of the
+   already overly-general CAP_SYS_ADMIN capability.
+ - If there is no related capability, then consider adding a new capability
+   bit -- but bear in mind that the numbering space is limited, and each new
+   bit needs to be understood and administered by sysadmins.

Perhaps mention alternative mechanisms for access control when working
on file descriptors, like avoiding security issues by looking at fd
_opener_ credentials, rather than current's credentials?

I'm struggling to cope up with text about this that doesn't feel either
too vague or much too detailed / internal, so maybe I'm misunderstanding
what you're after.  Could you clarify or maybe suggest a sentence or two?

Thanks,
David