On Wed, Mar 16, 2022 at 07:06:48PM +0000, Edgecombe, Rick P wrote: > On Tue, 2022-03-15 at 19:48 -0700, Kees Cook wrote: > > On Tue, Mar 15, 2022 at 09:53:13PM +0000, Edgecombe, Rick P wrote: > > > On Tue, 2022-03-15 at 13:41 -0700, Kees Cook wrote: > > > > Have you verified there's no binary difference in machine code > > > > output? > > > > > > There actually was a different in the binaries. I investigated a > > > bit, > > > and it seemed at least part of it was due to the line numbers > > > changing > > > the WARN_ON()s. But otherwise, I assumed some compiler optimization > > > must have been bumped. > > > > Right, you can ignore all the debugging line number changes. > > "diffoscope" should help see the difference by section. As long as > > the > > actual object code isn't changing, you should be good. > > What I did originally was objdump -D ptrace.o and diff that. Then I > slowly reduced changes to see what was generating the difference. When > I maintained the line numbers from the original version, and simply > converted the enum to defines, it still generated slightly different > code in places that didn't seem to connected to the changes. So I > figured the compiler was doing something, and relied on checking that > the actual constants didn't change in value. > > This morning I tried again to figure out what was causing the > difference. If I strip debug symbols, remove the BUILD_BUG_ON()s and > reformat the enums such that the line numbers are the same below the > enums then the objdump output is identical. > > I think what is happening in this debug stripped test, is that in the > call's to put_user(), it calls might_fault(), which has a __LINE__. > > But even adding a comment to the base file has surprisingly wide > effects. It caused the __bug_table section table to get code generated > with different instructions, not just line numbers constants changing. > > So I think there should be no functional change, but the binaries are > not identical. Right, that's fine: the instructions should be the same, just with various different offsets. -- Kees Cook
