Plugin makes 6 extra syscalls for every "gettimeofday" call

[stevenengler]

Whenever glibc tries to get the time, it seems to try to get timezone information first. It does this by reading "/etc/localtime", and in Shadow we handle this as a special-case that returns data from "/usr/share/zoneinfo/UTC" instead. This results in a lot of unnecessary extra syscalls. Every gettimeofday() call (which tor makes a lot of) results in several syscalls trying to mmap this file. All of the syscalls below (including the readlink) are related to the following gettimeofday sycall. These same syscalls are repeated every time that the plugin gets the current time.

00:00:01.000000000 [tid 1000] readlink(...) = <native>
00:00:01.000000000 [tid 1000] ^^^ = SysCallReg { as_i64: 27, as_u64: 27, as_ptr: shadow_shim_helper_rs::syscall_types::ForeignPtr<()> { val: 27 } }
00:00:01.000000000 [tid 1000] open("/etc/localtime", (empty), Mode(0x0)) = 3
00:00:01.000000000 [tid 1000] fstat() = 0
00:00:01.000000000 [tid 1000] mmap(0x0, 118, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7ffff7ffb000
00:00:01.000000000 [tid 1000] close(3) = 0
00:00:01.000000000 [tid 1000] munmap(0x7ffff7ffb000, 118) = <native>
00:00:01.000000000 [tid 1000] ^^^ = SysCallReg { as_i64: 0, as_u64: 0, as_ptr: shadow_shim_helper_rs::syscall_types::ForeignPtr<()> { val: 0 } }
000000001000000000 [tid 1000] gettimeofday(...) = 0

On my local machine, this only happens in tgen processes and not tor processes. On the benchmark runner, it seems to also happen for tor processes.

A workaround is to set the TZ=:/usr/share/zoneinfo/UTC environment variable for all processes. See https://blog.packagecloud.io/set-environment-variable-save-thousands-of-system-calls/. Then we get only:

000000023000000000 [tid 1000] gettimeofday(...) = 0

without any of the other syscalls above.

I'm not sure why glibc is doing all of this extra work, and how we can properly avoid it without requiring the user to configure an environment variable. We could maybe add the environment variable automatically, but that's not really a nice solution.