On 17.03.2022 11:02, Andrew Cooper wrote: > For livepatching, we need to look at a potentially clobbered function and > determine whether it used to have an ENDBR64 instruction. > > Use a non-default 4-byte P6 long nop, not emitted by toolchains, and extend > check-endbr.sh to look for it. > > The choice of nop has some complicated consequences. nopw (%rax) has a ModRM > byte of 0, which the Bourne compatible shells unconditionally strip from > parameters, meaning that we can't pass it to `grep -aob`.
Urgh. But as per my earlier comments I'm happier with ... > Therefore, use nopw (%rcx) so the ModRM byte becomes 1. ... a non-zero ModR/M byte anyway. > This then demonstrates another bug. Under perl regexes, \1 thru \9 are > subpattern matches, and not octal escapes. Switch the `grep -P` runes to use > hex escapes instead. > > The build time check then requires that the endbr64 poison have the same > treatment as endbr64 to avoid placing the byte pattern in immediate operands. > > Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com> Reviewed-by: Jan Beulich <jbeul...@suse.com> > Jan: As you had the buggy grep, can you please confirm that hex escapes work. (Build-)Tested-by: Jan Beulich <jbeul...@suse.com> When working out the workaround, I actually did test with hex, but then switched to octal to make easily visible that the two patterns actually match. I also did wonder about octal and sub-pattern matching conflicting, but the grep I used definitely didn't have an issue there. Hence I assume grep behavior changed at some point; I wonder how they mean to have octal expressed now. https://perldoc.perl.org/perlre specifically outlines how the conflict is dealt with - assuming you have observed grep to misbehave, I wonder whether they've accumulated a bug. (The doc also makes clear that such references aren't limited to single digit numbers; you may want to adjust your description in this regard.) Depending on how exactly your grep behaves, switching to always-three- digit octal escapes may be an alternative to retain the property of making obvious the similarity between the two pattern representations. Jan
