[GHC] #7823: ARM Thumb relocation fails to handle sign of veneer jumps

[GHC]

#7823: ARM Thumb relocation fails to handle sign of veneer jumps
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Reporter:  cjwatson       |          Owner:                  
    Type:  bug            |         Status:  new             
Priority:  normal         |      Component:  Runtime System  
 Version:  7.6.2          |       Keywords:                  
      Os:  Linux          |   Architecture:  Unknown/Multiple
 Failure:  Runtime crash  |      Blockedby:                  
Blocking:                 |        Related:                  
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 While working on http://hackage.haskell.org/trac/ghc/ticket/7794 on
 Linux/ARM (ARMv7, hard float), I found that 'inplace/bin/ghc-stage2
 --interactive' was raising various signals along the lines of
 SIGSEGV/SIGILL on startup.  Considerable hair-pulling and gdb later, I
 found that this was a jump into space when it should have been jumping to
 nl_langinfo from localeEncoding in base.  After even more poking around
 with gdb, I've tracked this down to a linker bug and have a patch.

 When the linker processes an R_ARM_THM_CALL or R_ARM_THM_JUMP24
 relocation, it sometimes needs to generate a "veneer" jump target if the
 true destination is out of range.  This is done almost entirely correctly.
 However, it reuses the sign bit decoded from the original instruction when
 re-encoding.  This means that if the veneer is on the opposite side of the
 branch instruction from the unrelocated target, the branch target will be
 encoded incorrectly.  This is a common case since the veneer is typically
 generated after the code being relocated but a to-be-relocated
 R_ARM_THM_CALL instruction will typically be a branch to PC-4 (a.k.a. the
 branch source itself).

 The simple, and I hope obvious, fix is to compute the sign from the new
 offset.  The attached patch should apply to both master and ghc-7.6.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7823>
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