Date: Fri, 25 Oct 2019 17:59:18 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Ian Lepore <ian@freebsd.org> Cc: freebsd-arm@freebsd.org Subject: Re: ucontext Message-ID: <20191025145918.GF73312@kib.kiev.ua> In-Reply-To: <d1bbda1f2a796d0a11b0feadbb9caf6004a2b40c.camel@freebsd.org> References: <20191024141133.04fb0693@i11.co> <20191024145436.GX73312@kib.kiev.ua> <20191025104421.012c1e5e@i11.co> <20191025083803.GD73312@kib.kiev.ua> <78c9868cf23643dfa2f88694542e86251bde13e7.camel@freebsd.org> <20191025144957.GE73312@kib.kiev.ua> <d1bbda1f2a796d0a11b0feadbb9caf6004a2b40c.camel@freebsd.org>
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On Fri, Oct 25, 2019 at 08:51:41AM -0600, Ian Lepore wrote: > On Fri, 2019-10-25 at 17:49 +0300, Konstantin Belousov wrote: > > On Fri, Oct 25, 2019 at 08:26:19AM -0600, Ian Lepore wrote: > > > On Fri, 2019-10-25 at 11:38 +0300, Konstantin Belousov wrote: > > > > On Fri, Oct 25, 2019 at 10:44:21AM +0300, Nick Kostirya wrote: > > > > > On Thu, 24 Oct 2019 17:54:36 +0300 > > > > > Konstantin Belousov <kostikbel@gmail.com> wrote: > > > > > > > > > > > > > > > > > I believe you want > > > > > > uc_context.__gregs[_REG_PC] > > > > > > on arm (32bit) and > > > > > > uc_context.mc_gpregs.gp_elr > > > > > > on arm64 for aarch64. > > > > > > > > > > > > Sometimes the thumb bit (lowest bit in PC) leaks there, then > > > > > > you should > > > > > > mask it. > > > > > > > > > > Thanks! > > > > > > > > > > Although I did not understand your last phrase. > > > > > There is leak of what? > > > > > > > > Leak of the thumb bit. ARM ARM specifies that in non-thumb mode, > > > > pc must > > > > be word-aligned, in thumb it is half-word aligned. A way to > > > > enter thumb > > > > mode is to execute BX or BLX instruction with the lowest bit of > > > > the target > > > > PC set to 1. > > > > > > > > Sometimes you might get pc with the bit 0 set, which should > > > > be masked out then. This is a bigger issue for unwinders than > > > > for simple > > > > profilers. > > > > > > > > > Where can I read about it? > > > > > > > > ARM ARM (ARM architecture reference manual), available from > > > > arm.com. > > > > Or Google for it. > > > > > > > > > > The kernel has some support for running thumb binaries, but I've > > > never > > > heard of anybody actually doing so on freebsd. Nobody has ever > > > reported a bug related to running a thumb binary, and it would be > > > astounding to me if we accidentally got everything in the kernel > > > thumb > > > support right on the first try without ever testing it. > > > > I am curious as well, isn't thumb completely transparent to the > > kernel ? > > I.e. my impression was that some code might be compiled into thumb, > > and > > then a thunk which does BX to the location, is used to switch to > > thumb > > mode. There is no new ELF machine type involved, or different > > exception > > entry mode, so it should just work ? > > > > And this is why I remember about this bit 0 issue, it caused some > > problems > > to libunwind on arm. > > > > I think in the kernel it would appear in places like page fault > handlers needing to mask off the lower bit. Normally thumb state is stored in PSTATE.T and not in R15. Also, even if it would be R15.0, why would kernel need it masked ?
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