On Fri, Aug 21, 2020 at 1:08 PM Bae, Chang Seok <chang.seok....@intel.com> wrote: > > > > On Aug 20, 2020, at 21:41, Kyle Huey <m...@kylehuey.com> wrote: > > > > On the x86-64 5.9-rc1 TLS is completely broken in 32 bit tracees when > > running under rr[0]. Booting the kernel with `nofsgsbase` fixes it and > > I bisected to > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.8&id=673903495c85137791d5820d690229efe09c8f7b. > > > > STR: > > 1. Build rr from source by > > a. git clone https://github.com/mozilla/rr > > b. mkdir rr/obj > > c. cd rr/obj > > d. cmake .. > > e. make -j16 > > 2. Run the simple 32 bit tracee outside of rr with `./bin/simple_32`. > > It should print a message and exit cleanly. > > 3. Run it under rr with `./bin/rr ./bin/simple_32`. > > > > It should behave the same way, but with fsgsbase enabled it will > > segfault. The `simple_32` binary is a simple "hello world" type > > program but it does link to pthreads, so pre-main code attempts to > > access TLS variables. > > > > The interplay between 32 bit and 64 bit TLS is dark magic to me > > unfortunately so this is all the useful information I have. > > As I run it and collect the ptrace logs, it starts to set FSBASE with > some numbers, e.g. 140632147826496, and then later attempts to set GS > with 99 through putreg(), not putreg32(). > > With FSGSBASE, the FS/GS base is decoupled from the selector. Andy > made putreg32() to retain the old behavior, fetching FS/GS base > according to the index, but the putreg() does not do. So, rr probably > relies on the old behavior as observed to setting the GS index only. > But it was previously considered to be okay with this comment [1]: > > "Our modifications to fs/gs/fs_base/gs_base are always either a) > setting values that the kernel set during recording to make them > happen during replay or b) emulating PTRACE_SET_REGS that a tracee > ptracer tried to set on another tracee. Either way I think the > effects are going to be the same as what would happen if the > program were run without rr." > > It is not straightforward to go into the rr internal to me. Robert, > any thought?
Hmm. When we are running a 32 bit tracee in a 64 bit build of rr we internally convert between the 32 bit and 64 bit user_regs_structs[0] at the ptrace boundary. This does not preserve the fs/gsbase (because there is no fs/gsbase in the 32 bit user_regs_struct, of course). 40c45904f818c1f6555294ca27afc5fda4f09e68 added magic for a 32 bit tracer tracing a 32 bit tracee on a 64 bit kernel, but it looks like a 64 bit tracer tracing a 32 bit tracee on a 64 bit kernel *is* now expected to preserve the fs/gsbase values (or die, in our case). Is that correct? - Kyle [0] https://github.com/mozilla/rr/blob/fcd2a26680a3fc2bda5f40d732d0c72b9628357b/src/Registers.cc#L519
