On Mon, Jan 15, 2018 at 05:49:47PM +0000, Russell King - ARM Linux wrote: > On Thu, Jan 11, 2018 at 06:59:37PM -0600, Eric W. Biederman wrote: > > Setting si_code to 0 results in a userspace seeing an si_code of 0. > > This is the same si_code as SI_USER. Posix and common sense requires > > that SI_USER not be a signal specific si_code. As such this use of 0 > > for the si_code is a pretty horribly broken ABI. > > > > Further use of si_code == 0 guaranteed that copy_siginfo_to_user saw a > > value of __SI_KILL and now sees a value of SIL_KILL with the result > > that uid and pid fields are copied and which might copying the si_addr > > field by accident but certainly not by design. Making this a very > > flakey implementation. > > > > Utilizing FPE_FIXME, siginfo_layout will now return SIL_FAULT and the > > appropriate fields will be reliably copied. > > So what do you suggest when none of the SIGFPE FPE_xxx codes match the > condition that "we don't know what happened" ? Raise a SIGKILL instead > maybe? We will have dumped the VFP state into the kernel log at this > point, things are pretty much fscked. > > It's probably an impossible condition unless the hardware has failed, > no one has knowingly reported getting such a dump in their kernel log, > so it's something that could very likely be changed in some way > without anyone noticing. arm64 can optionally not tell you exactly what exception(s) happened for vector operations. This may also be true for 32-bit. Unfortunately, there's nothing sensible to report in this case, and there is no such constant as FPE_UNKNOWN. Currently arm64 sets si_code to 0, which doesn't match any FPE_* constant (but unfortunately matches SI_USER) -- defining a new, distinct FPE_UNKNOWN is probably no worse than that. I'm not sure yet whether a similar argument applies for 32-bit. Cheers ---Dave
