On Mon, 6 May 2019 20:05:24 -0700 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > It would emulate the call that has had its first byte overwritten by > 'int3'. Without doing any lookups of what it was supposed to change > the call to, because it simply depends on what the rewriting code is > doing on another CPU (or on the same CPU - it wouldn't care). OK, so this is just about what to have it call. > > So no need to look up anything, not at int3 time, and not at return > time. It would just emulate the instruction atomically, with no state, > and no need to look up what the 'ip' instruction is at the time. > > It could literally just use a single flag: "is ftrace updating call > instructions". Add another flag for the "I'm nop'ing out call > instructions" so that it knows to emulate a jump-over instead. That's > it. Well we have that, and we have to look up the record regardless to know if this was a ftrace int3 or not (the ftrace_location(ip) does that). And the record has a counter to # of attached callers. Zero being to turn it into a nop. Note, if we are going from nop to call or call to nop, it would need to read the offset to see if it is a nop (don't want to call with the nop offset) > > Because all the actual *values* would be entirely be determined by the > actual rewriting that is going on independently of the 'int3' > exception. But still, we need to emulate the call, which requires pushing the return code back onto the stack. I believe that part is the part we are struggling with. -- Steve
