Date: Fri, 25 Oct 2019 08:26:19 -0600 From: Ian Lepore <ian@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com>, Nick Kostirya <nikolay.kostirya@i11.co> Cc: freebsd-arm@freebsd.org Subject: Re: ucontext Message-ID: <78c9868cf23643dfa2f88694542e86251bde13e7.camel@freebsd.org> In-Reply-To: <20191025083803.GD73312@kib.kiev.ua> References: <20191024141133.04fb0693@i11.co> <20191024145436.GX73312@kib.kiev.ua> <20191025104421.012c1e5e@i11.co> <20191025083803.GD73312@kib.kiev.ua>
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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. -- Ian
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