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. - Kyle [0] https://rr-project.org/
