ocamlopt -nodynlink does not work on armhf

[glondu]

Hello,

As instructed in ocaml/dune#2527, I open an issue here.

When I compile a simple hello.ml file (Debian unstable armhf):

print_endline "Hello world!"

with ocamlopt -nodynlink hello.ml, I get relocation errors that go away when I remove -nodynlink.

Please have look at @nojb's insights in ocaml/dune#2527

[nojb]

It seems (https://sourceware.org/ml/binutils/2009-04/msg00395.html) we should not be using MOVW/MOVT to load addresses of symbols coming from PIC libraries :

ocaml/asmcomp/arm/emit.mlp

Lines 389 to 392 in 0557f07

	end else if !arch > ARMv6 && not !Clflags.dlcode && !fastcode_flag then begin
	` movw {emit_reg dst}, #:lower16:{emit_symbol s}\n`;
	` movt {emit_reg dst}, #:upper16:{emit_symbol s}\n`;
	2

[github-actions]

This issue has been open one year with no activity. Consequently, it is being marked with the "stale" label. What this means is that the issue will be automatically closed in 30 days unless more comments are added or the "stale" label is removed. Comments that provide new information on the issue are especially welcome: is it still reproducible? did it appear in other contexts? how critical is it? etc.

[xavierleroy]

A friendly bot brought this issue to my attention, one year later.

Generally speaking, ocamlopt -c -nodynlink produces code appropriate for statically-linked executables. However, OS distributions are increasingly using position-independent executables (PIE), which are not statically linked and undergo dynamic relocation at every execution.

Here I can reproduce the problem on my Raspbian rPi by using ocamlopt -ccopt -pie -nodynlink. I guess Raspbian doesn't use PIE by default yet. Likewise, I guess @glondu can make the problem go away by using ocamlopt -ccopt -no-pie -nodynlink.

We need to discuss the future of the -nodynlink option. Should it go away? Should it turn PIE off at link-time? Should it be restricted so that the generated code is still compatible with PIE even though it may not be PIC?

For ARM, not using MOVW/MOVT as suggested in #8867 (comment) would implement the third approach above, and would not have much performance impact, I guess. However, I need to check what happens with other non-x86 platforms.

[shindere]

@Armael may be interested in following this

[xavierleroy]

Results of a quick experiment: -nodynlink is incompatible with PIE on AMD64 (x86 64 bits) and ARM (32 bits). In both cases, an error is reported at link-time.

For AMD64, PIC code is generated by default, which trumps -nodynlink, so one needs to do ocamlopt -nodynlink -fno-PIC to run into the issue.

ARM64, Power, and I386 (x86 32 bits) seem OK. s390x (Z systems) doesn't support PIE at all. RISC-V could not be tested.

The appropriate action, I think, is to document that -nodynlink is incompatible with PIE and requires -ccopt -no-pie on systems that create PIEs by default.