Dave P Martin <Dave.Martin@xxxxxxx> writes: > On Thu, May 23, 2019 at 03:53:06PM +0100, Eric W. Biederman wrote: >> Dave Martin <Dave.Martin@xxxxxxx> writes: >> >> > On Thu, May 23, 2019 at 01:38:53AM +0100, Eric W. Biederman wrote: >> >> It really only matters to debuggers but the SIGKILL does not have any >> >> si_codes that use the fault member of the siginfo union. Correct this >> >> the simple way and call force_sig instead of force_sig_fault when the >> >> signal is SIGKILL. >> > >> > I haven't fully understood the context for this, but why does it matter >> > what's in siginfo for SIGKILL? My understanding is that userspace >> > (including ptrace) never gets to see it anyway for the SIGKILL case. >> >> Yes. In practice I think it would take tracing or something very >> exotic to notice anything going wrong because the task will be killed. >> >> > Here it feels like SIGKILL is logically a synchronous, thread-targeted >> > fault: we must ensure that no subsequent insn in current executes (just >> > like other fault signal). In this case, I thought we fall back to >> > SIGKILL not because there is no fault, but because we failed to >> > properly diagnose or report the type of fault that occurred. >> > >> > So maybe handling it consistently with other faults signals makes >> > sense. The fact that delivery of this signal destroys the process >> > before anyone can look at the resulting siginfo feels like a >> > side-effect rather than something obviously wrong. >> > >> > The siginfo is potentially useful diagnostic information, that we could >> > subsequently provide a means to access post-mortem. >> > >> > I just dived in on this single patch, so I may be missing something more >> > fundamental, or just being pedantic... >> >> Not really. I was working on another cleanup and this usage of SIGKILL >> came up. >> >> A synchronous thread synchronous fault gets us as far as the forc_sig >> family of functions. That only leaves the question of which union >> member in struct siginfo we are using. The union members are _kill, >> _fault, _timer, _rt, _sigchld, _sigfault, _sigpoll, and _sigsys. >> >> As it has prove quite error prone for people to fill out struct siginfo >> in the past by hand, I have provided a couple of helper functions for >> the common cases that come up such as: force_sig_fault, >> force_sig_mceerr, force_sig_bnderr, force_sig_pkuerr. Each of those >> helper functions takes the information needed to fill out the union >> member of struct siginfo that kind of fault corresponds to. >> >> For the SIGKILL case the only si_code I see being passed SI_KERNEL. >> The SI_KERNEL si_code corresponds to the _kill union member while >> force_sig_fault fills in fields for the _fault union member. >> >> Because of the mismatch of which union member SIGKILL should be using >> and the union member force_sig_fault applies alarm bells ring in my head >> when I read the current arm64 kernel code. Somewhat doubly so because >> the other fields in passed to force_sig_fault appear to be somewhat >> random when SIGKILL is the signal. >> >> So I figured let's preserve the usage of SIGKILL as a synchronous >> exception. That seems legitimate and other folks do that as well but >> let's use force_sig instead of force_sig_fault instead. I don't know if >> userspace will notice but at the very least we won't be providing a bad >> example for other kernel code to follow and we won't wind up be making >> assumptions that are true today and false tomorrow when some >> implementation detail changes. >> >> For imformation on what signals and si_codes correspond to which >> union members you can look at siginfo_layout. That function >> is the keeper of the magic decoder key. Currently the only two >> si_codes defined for SIGKILL are SI_KERNEL and SI_USER both of which >> correspond to a _kill union member. > > I see. Assuming we cannot have a dummy internal si_code for this > special case (probably a bad idea), I think Will's suggestion of at > least pushing the special case handling down into > arm64_force_sig_fault() is probably a bit cleaner here, expecially > if other callers of that function may pass in SIGKILL (I haven't > looked though). Done in my v2 version of this patch. Eric
