runtime: sigaction is calling getg before TLS is set up on Android

[eliasnaur]

The following crash has started to appear when running Android apps with Go tip:

04-15 20:49:36.875  4033  4033 F DEBUG   :     #00 pc 00000000000a978c  /data/app/com.example.BindTest.test-iyBWQdBs046wze2Z_WqATA==/lib/x86_64/libgojni.so (runtime.sigaction+108)
04-15 20:49:36.875  4033  4033 F DEBUG   :     #01 pc 00000000000e4432  /data/app/com.example.BindTest.test-iyBWQdBs046wze2Z_WqATA==/lib/x86_64/libgojni.so (runtime.initsig+146)
04-15 20:49:36.875  4033  4033 F DEBUG   :     #02 pc 00000000000ced96  /data/app/com.example.BindTest.test-iyBWQdBs046wze2Z_WqATA==/lib/x86_64/libgojni.so (runtime.libpreinit+22)
04-15 20:49:36.875  4033  4033 F DEBUG   :     #03 pc 00000000000fb32a  /data/app/com.example.BindTest.test-iyBWQdBs046wze2Z_WqATA==/lib/x86_64/libgojni.so (_rt0_amd64_lib+74)
04-15 20:49:36.875  4033  4033 F DEBUG   :     #04 pc 0000000000025b4f  /system/bin/linker64 (__dl__ZL10call_arrayIPFviPPcS1_EEvPKcPT_mbS5_+255)
04-15 20:49:36.875  4033  4033 F DEBUG   :     #05 pc 0000000000000000  <unknown>

I believe the crash is caused by an invalid g returned from getg, called from sigaction. At the time libpreinit is called, TLS hasn't been set up yet (it is set up later in rt0_go).

This started to appear after my fixes for #29674 but I believe it's been a latent bug for a long time. We've just been lucky that the static TLS slot previously used for accessing g on android/amd64 and android/386 always contained 0.

The cause is clear but the fix isn't obvious to me. Please advice, @cherrymui @ianlancetaylor .

[ianlancetaylor]

I assume that the problem is the call to getg in the version of sigaction in runtime/cgo_sigaction.go? I think the fix would be something like

    var g *g
    if mainStarted {
        g = getg()
    }

[gopherbot]

Change https://golang.org/cl/172158 mentions this issue: runtime: avoid getg in preinit on Android