runtime: modify_ldt is optional in newer linux kernels, use set_thread_area instead

[minux]

see https://lkml.org/lkml/2015/7/21/759

we probably need to switch to use set_thread_area(2) instead for linux/386.
(do we need to apply this Go 1.4.4 so that Go 1.4 continues to run on
newer kernels with disabled modify_ldt? tentatively labeled Go1.4.4)

According to arch_prctl(2) man page: http://man7.org/linux/man-pages/man2/arch_prctl.2.html
Context switches for 64-bit segment bases are rather expensive. As
an optimization, if a 32-bit TLS base address is used, arch_prctl(2)
may use a real TLS entry as if set_thread_area(2) had been called,
instead of manipulating the segment base register directly.

so we probably could switch to set_thread_area even on linux/amd64,
but I see that glibc still uses arch_prctl(ARCH_SET_FS), so probably
the overhead is not that big a problem.

[stanaka]

I've got an error, Trace/breakpoint trap with a crossbuilt 386 binary on Amazon Linux 2016.03, which is released a few days ago. This issue probably caused the error.

I've confirmed it on the following environment.

$ go version
go version go1.6 linux/amd64
$ uname -a
Linux ip-10-1-8-251 4.4.5-15.26.amzn1.x86_64 #1 SMP Wed Mar 16 17:15:34 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

[djtm]

You can also try to echo 1 > /proc/sys/modify_ldt as workaround. But this has security implications.

[stanaka]

Unfortunately the kernel config CONFIG_MODIFY_LDT_SYSCALL of Amazon Linux 2016.03 is disabled, so /proc/sys/modify_ldt does not exist. ( /proc/sys/kernel/modify_ldt also does not exist. )

$ cat /boot/config-4.4.5-15.26.amzn1.x86_64  | grep MODIFY_LDT
# CONFIG_MODIFY_LDT_SYSCALL is not set

[stanaka]

With this patch, https://gist.github.com/stanaka/2b63a16d484692b3599f, which is based on the patch on this comment #386 (comment), linux/386 binary seems to work well on Amazon Linux 2016.03, of which kernel is 4.4.5.

[gopherbot]

CL https://golang.org/cl/21190 mentions this issue.