cmd/link: remove text relocations from android libraries

[gopherbot]

by momchil.dev:

What does 'go version' print?

go version go1.4rc2 darwin/amd64

What happened?

I am compiling a shared library for android/arm. I then use this library from a
Java-based android application. 

When I run the application on a Nexus 5 device with Lollipop, I get a
"libexample.so has text relocations. This is wasting memory and prevents security
hardening. Please fix."

If I run the same application on an old Kitkat Asus Transformer Pad TF301T, I don't get
any errors at all and everything works as expected.

Please provide any additional information below.

I don't have much experience in developing native libraries but the fact that it works
on one device and not the other seems strange.

You can find the code for the library here:
https://github.com/momchil-atanasov/go-android-lib
You can find the code for the android app here:
https://github.com/momchil-atanasov/go-android-app

Hope this helps! Can't wait for the official release of 1.4.

[crawshaw]

I'm not sure what's involved in fixing this. Assigning it to myself to work out if it can be done for the 1.5 release.

[minux]

I don't have NDK installed, could someone please attach a .so file compiled
by Go that triggers text relocation warnings? I want to take a look.

[gen2brain]

Here it is, I also saw this error.
http://sandbox.blokada.in.rs/libgojni.so

[ianlancetaylor]

Actually this probably needs to be fixed in cmd/ld.

What is the output of "readelf -r" on the .so for which you are getting the warning?

[gen2brain]

https://gist.githubusercontent.com/gen2brain/17f90af8f266796d26d6/raw/readelf

[ianlancetaylor]

Thanks. There are no text relocations in that list, so I am perplexed by the error message you reported.

[gen2brain]

Actually, I didn't opened this issue, but I saw same message as momchil.dev when running in lollipop emulator.

[minux]

Looking at the readelf -r report more closely, there are indeed a lot of
text relocations. It's just the section name is .rel.dyn instead of .rel.text.

For example, the first entry of .rel.dyn is:
Offset Info Type Sym.Value Sym. Name
0010cb8c 00000017 R_ARM_RELATIVE

And the .text section does include that address:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 8] .text PROGBITS 001071d8 1071d8 271a8c 00 AX 0 0 4

Looking at the disassembly, we find out that there is indeed an absolute
address at 0x10cb8c:
10cb7c: e59fb008 ldr fp, [pc, #8] ; 10cb8c
10cb80: e5cb0000 strb r0, [fp]
10cb84: e49df004 pop {pc} ; (ldr pc, [sp], #4)
10cb88: eafffffe b 10cb88
10cb8c: 00589d6f ;; <--- this word is being relocated

It seems cmd/5l needs to know how to use GOT to access global data even for
Go data when -shared is active.

However, as the code is laid out by 5g, and 5g doesn't know the link mode
yet, I don't know how to easily solve this problem. Pass -shared to 5g?
This is probably too invasive.

Suggestions?

[ianlancetaylor]

Thanks for checking me on the relocations.

In order to add full shared library support as outlined in https://docs.google.com/a/golang.org/document/d/1nr-TQHw_er6GOQRsF6T43GGhFDelrAP0NqSS_00RgZQ/edit#heading=h.fwmrrio0df0i I think that we are going to have to start passing a special option to all the gc compilers.

[jhleath]

So, does this issue imply that Android support is currently unusable for Nexus devices?

[crawshaw]

I don't think that is implied. The android platform wants us to avoid text relocations, and we should move in that direction. But apps are producing a log warning, not an error.