[Bug 725126]

[Jason A. Donenfeld]

Tracking the new bug here now:
https://sourceware.org/bugzilla/show_bug.cgi?id=26141

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https://bugs.launchpad.net/bugs/725126

Title:
  gas may assemble b to locally-defined, preemptible global symbol as
  "b.n"

Status in binutils:
  Fix Released
Status in Linaro Binutils:
  Won't Fix
Status in Cortex String Routines:
  Invalid
Status in armel-cross-toolchain-base package in Ubuntu:
  Fix Released
Status in binutils package in Ubuntu:
  Fix Released

Bug description:
  There's no guarantee that R_ARM_THM_JUMP11 relocation can be resolved
  correctly if the symbol gets preempted.

  Observed in binutils-arm-linux-gnueabi 2.20.51.20100908-0ubuntu2cross1.52.
  Observed on binutils trunk on 2011-02-25.

  The problem seems to strike when the compiler optimises a sibling call
  to "b <label>", for which the assembler generates a b.n.

  As a side-effect of the presence of this relocation, Thumb-2 kernel
  modules may fail to load, since the kernel doesn't support fixing up
  this relocation.  I believe the kernel doesn't do any symbol
  preemption processing when loading modules -- if so, the relocation is
  actually redundant.  But there's no way for the kernel to detect at
  module load time that the relocation can safely be ignored.  In kernel
  builds, about 1 out every 6 modules suffers from this problem.  It's
  not clear to me whether the method used by the kernel to link .ko
  objects is wrong or not.

  It seems that either the tools should support symbol preemption in
  this scenario (implying that a short branch is not adequate to
  guarantee that preemption will work -- i.e., a gas bug) or not (in
  this case the fixup should be done locally and there should presumably
  be no relocation -- i.e., a different gas bug).

  It appears that passing -fno-optimize-sibling-calls to GCC can work
  around the problem by avoiding the generation of local branches to
  global symbols, but this is clearly not the correct fix.

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