Re: [lkp] [+573 bytes kernel size regression] [i386-tinyconfig] [00f2dfd8c2] open: introduce openat2(2) syscall

[Josh Triplett <josh@joshtriplett.org>]

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On Wed, Nov 06, 2019 at 01:44:14AM +1100, Aleksa Sarai wrote:
> On 2019-11-05, kbuild test robot <lkp@intel.com> wrote:
> > 
> > FYI, we noticed a +573 bytes kernel size regression due to commit:
> > 
> > commit: 00f2dfd8c2a40f6dd7a74eae598a174484d848c4 (open: introduce openat2(2) syscall)
> >  
> > 
> > Details as below (size data is obtained by `nm --size-sort vmlinux`):
> > 
> > 97a4cce0: namei: LOOKUP_{IN_ROOT,BENEATH}: permit limited ".." resolution
> > 00f2dfd8: open: introduce openat2(2) syscall
> > 
> > +-----------------------+----------+----------+-------+
> > |        symbol         | 97a4cce0 | 00f2dfd8 | delta |
> > +-----------------------+----------+----------+-------+
> > | bzImage               | 439168   | 439424   | 256   |
> > | nm.t.build_open_flags | 214      | 349      | 135   |
> > | nm.T.__se_sys_openat2 | 0        | 122      | 122   |
> > | nm.T.sys_openat2      | 0        | 122      | 122   |
> > | nm.t.do_sys_openat2   | 0        | 116      | 116   |
> > | nm.t.build_open_how   | 0        | 78       | 78    |
> > | nm.T.file_open_name   | 41       | 73       | 32    |
> > | nm.T.file_open_root   | 52       | 84       | 32    |
> > | nm.D.sys_call_table   | 1744     | 1752     | 8     |
> > | nm.T.__se_sys_openat  | 26       | 23       | -3    |
> > | nm.T.sys_openat       | 26       | 23       | -3    |
> > | nm.T.__se_sys_open    | 27       | 24       | -3    |
> > | nm.T.sys_open         | 27       | 24       | -3    |
> > | nm.T.do_sys_open      | 121      | 61       | -60   |
> > +-----------------------+----------+----------+-------+
> 
> I'm not sure I understand what is meant by "regression" in this context.
> Surely new code will always result in a code size increase. Or is it
> that the largest symbols are increasing in size? Is there something I
> should do, or is it reasonable that the size has increased (given my
> changes in build_open_flags)?

It would be nice if (for instance) it was possible to compile out the
old openat and *only* support openat2, or otherwise compile out unused
syscalls. In general, we're watching for new code that can't be built
out of the kernel even in "allnoconfig"/"tinyconfig".