ocaml-maint team mailing list archive

[Dave Martin <dave.martin@xxxxxxxxxx>]

It looks like ARM code is being executed as Thumb.

The .s file assembled as ARM because that's always the default if there
are no .thumb directives and no -mthumb command-line option.  (It would
need some minor changes to work in Thumb anyway).  However, the CPSR
value when we segfault indicates that the processor is executing in
Thumb state ((cpsr & 0x20) == 0x20)

We end up in the wrong state because the call originating from
caml_main() in the linked program calls caml_start_program with a "bl"
instruction, so there is no switch from Thumb to ARM.  As a result, we
execute garbage at caml_start_program.

This may indicate a bug in the linker, but since none of the symbols in
the startup .s file are marked as function symbols, it might be that the
linker isn't absolutely required to perform the correct fixup in this
case.

Can you try adding a ".type <function name>, %function" directive for
each function in the startup .s file?

If assembling and linking the resulting object results in a working
program, this suggests that the linker is sensitive to the symbol type
when doing branch fixups.

If so, we can work around it my adding all the required .type directives
for code symbols in ocaml, but we should still raise the issue as a
possible linker bug.

(gdb) r
Starting program: /mnt/a.out 

Program received signal SIGSEGV, Segmentation fault.
0x00014f94 in caml_start_program ()
(gdb) i r
r0             0x0      0
r1             0x0      0
r2             0xb8d7   47319
r3             0x0      0
r4             0xbefff9c4       3204446660
r5             0x4      4
r6             0xbefff8c4       3204446404
r7             0x0      0
r8             0x0      0
r9             0x0      0
r10            0x4001e000       1073864704
r11            0x0      0
r12            0x400b98c1       1074501825
sp             0xbefff748       0xbefff748
lr             0x153b1  86961
pc             0x14f94  0x14f94 <caml_start_program>
cpsr           0x40000030       1073741872
(gdb)

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https://bugs.launchpad.net/bugs/810402

Title:
  all native ocaml programs segfault on armel

Status in Linaro Binutils:
  New
Status in “binutils” package in Ubuntu:
  Confirmed
Status in “ocaml” package in Ubuntu:
  Confirmed
Status in “binutils” source package in Oneiric:
  Confirmed
Status in “ocaml” source package in Oneiric:
  Confirmed

Bug description:
  Starting from version 2.21.52.20110606-1ubuntu1 (in oneiric), all
  programs produced by ocamlopt segfault on Ubuntu/armel. The problem
  disappears after downgrading binutils to 2.21.51.20110421-6ubuntu1.

  Steps to reproduce: echo > empty.ml && ocamlopt empty.ml && ./a.out

  Using the "-S" option of ocamlopt might be useful, to have a look at
  the generated assembly code, but I don't know what to look for... I
  hope someone more familiar with binutils code can help.

  I found that while investigating the recent build failures of ocaml
  stuff or armel. Everything works find in the armel port of Debian.

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