Re: [ARM/FDPIC v2 09/21] [ARM] FDPIC: Add support for taking address of nested function

[Christophe Lyon <christophe dot lyon at linaro dot org>]

On Wed, 29 Aug 2018 at 13:01, Kyrill Tkachov
<kyrylo.tkachov@foss.arm.com> wrote:
>
> Hi Christophe,
>
> On 13/07/18 17:11, christophe.lyon@st.com wrote:
> > From: Christophe Lyon <christophe.lyon@linaro.org>
> >
> > In FDPIC mode, the trampoline generated to support pointers to nested
> > functions looks like:
> >
> >            .word trampoline address
> >            .word trampoline GOT address
> >            ldr            r12, [pc, #8]
> >            ldr            r9, [pc, #8]
> >            ldr           pc, [pc]
>
> The comment in the code says the last one is:
> ldr           pc,  [pc, #8] ; #4 for Thumb2
>
> I'm assuming the code one is correct.

Right, it looks like a typo in the commit message.

>
> >            .word static chain value
> >            .word GOT address
> >            .word function's address
> >
> > because in FDPIC function pointers are actually pointers to function
> > descriptors, we have to actually generate a function descriptor for
> > the trampoline.
> >
> > 2018-XX-XX  Christophe Lyon  <christophe.lyon@st.com>
> >         Mickaël Guêné <mickael.guene@st.com>
> >
> >         gcc/
> >         * config/arm/arm.c (arm_asm_trampoline_template): Add FDPIC
> >         support.
> >         (arm_trampoline_init): Likewise.
> >         (arm_trampoline_init): Likewise.
> >         * config/arm/arm.h (TRAMPOLINE_SIZE): Likewise.
> >
> > Change-Id: I4b5127261a9aefa0f0318f110574ec07a856aeb1
> >
> > diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
> > index 51da2bc..ffc9128 100644
> > --- a/gcc/config/arm/arm.c
> > +++ b/gcc/config/arm/arm.c
> > @@ -3950,13 +3950,50 @@ arm_warn_func_return (tree decl)
> >             .word static chain value
> >             .word function's address
> >     XXX FIXME: When the trampoline returns, r8 will be clobbered.  */
> > +/* In FDPIC mode, the trampoline looks like:
> > +          .word trampoline address
> > +          .word trampoline GOT address
> > +          ldr            r12, [pc, #8] ; #4 for Thumb2
> > +          ldr            r9,  [pc, #8] ; #4 for Thumb2
> > +          ldr           pc,  [pc, #8] ; #4 for Thumb2
> > +          .word static chain value
> > +          .word GOT address
> > +          .word function's address
> > +*/
> >  static void
> >  arm_asm_trampoline_template (FILE *f)
> >  {
> >    fprintf (f, "\t.syntax unified\n");
> >
> > -  if (TARGET_ARM)
> > +  if (TARGET_FDPIC)
> > +    {
> > +      /* The first two words are a function descriptor pointing to the
> > +        trampoline code just below.  */
> > +      if (TARGET_ARM)
> > +       fprintf (f, "\t.arm\n");
> > +      else if (TARGET_THUMB2)
> > +       fprintf (f, "\t.thumb\n");
> > +      else
> > +       /* Only ARM and Thumb-2 are supported.  */
> > +       gcc_assert ( !TARGET_ARM && !TARGET_THUMB2);
> > +
>
> This cannot trigger based on the two clauses above. I think you want to just make it gcc_unreachable ().
>
OK

> > +      assemble_aligned_integer (UNITS_PER_WORD, const0_rtx);
> > +      assemble_aligned_integer (UNITS_PER_WORD, const0_rtx);
> > +      /* Trampoline code which sets the static chain register but also
> > +        PIC register before jumping into real code.  */
> > +      asm_fprintf (f, "\tldr\t%r, [%r, #%d]\n",
> > +                  STATIC_CHAIN_REGNUM, PC_REGNUM,
> > +                  TARGET_THUMB2 ? 8 : 4);
> > +      asm_fprintf (f, "\tldr\t%r, [%r, #%d]\n",
> > +                  PIC_OFFSET_TABLE_REGNUM, PC_REGNUM,
> > +                  TARGET_THUMB2 ? 8 : 4);
> > +      asm_fprintf (f, "\tldr\t%r, [%r, #%d]\n",
> > +                  PC_REGNUM, PC_REGNUM,
> > +                  TARGET_THUMB2 ? 8 : 4);
> > +      assemble_aligned_integer (UNITS_PER_WORD, const0_rtx);
> > +    }
> > +  else if (TARGET_ARM)
> >      {
> >        fprintf (f, "\t.arm\n");
> >        asm_fprintf (f, "\tldr\t%r, [%r, #0]\n", STATIC_CHAIN_REGNUM, PC_REGNUM);
> > @@ -3997,12 +4034,37 @@ arm_trampoline_init (rtx m_tramp, tree fndecl, rtx chain_value)
> >    emit_block_move (m_tramp, assemble_trampoline_template (),
> >                     GEN_INT (TRAMPOLINE_SIZE), BLOCK_OP_NORMAL);
> >
> > -  mem = adjust_address (m_tramp, SImode, TARGET_32BIT ? 8 : 12);
> > -  emit_move_insn (mem, chain_value);
> > +  if (TARGET_FDPIC)
> > +    {
> > +      rtx funcdesc = XEXP (DECL_RTL (fndecl), 0);
> > +      rtx fnaddr = gen_rtx_MEM (Pmode, funcdesc);
> > +      rtx gotaddr = gen_rtx_MEM (Pmode, plus_constant (Pmode, funcdesc, 4));
> > +      rtx trampoline_code_start
> > +       = plus_constant (Pmode, XEXP (m_tramp, 0), TARGET_THUMB2 ? 9 : 8);
>
> 9? Can you comment on this value?
>
The function start address is a offset 8, but in Thumb mode we want
bit 0 set to 1 to indicate thumb-ness.

> > +
> > +      /* Write initial funcdesc which points to the trampoline.  */
> > +      mem = adjust_address (m_tramp, SImode, 0);
> > +      emit_move_insn (mem, trampoline_code_start);
> > +      mem = adjust_address (m_tramp, SImode, 4);
> > +      emit_move_insn (mem, gen_rtx_REG (Pmode, PIC_OFFSET_TABLE_REGNUM));
> > +      /* Setup static chain.  */
> > +      mem = adjust_address (m_tramp, SImode, 20);
> > +      emit_move_insn (mem, chain_value);
> > +      /* GOT + real function entry point.  */
> > +      mem = adjust_address (m_tramp, SImode, 24);
> > +      emit_move_insn (mem, gotaddr);
> > +      mem = adjust_address (m_tramp, SImode, 28);
> > +      emit_move_insn (mem, fnaddr);
> > +    }
> > +  else
> > +    {
> > +      mem = adjust_address (m_tramp, SImode, TARGET_32BIT ? 8 : 12);
> > +      emit_move_insn (mem, chain_value);
> >
> > -  mem = adjust_address (m_tramp, SImode, TARGET_32BIT ? 12 : 16);
> > -  fnaddr = XEXP (DECL_RTL (fndecl), 0);
> > -  emit_move_insn (mem, fnaddr);
> > +      mem = adjust_address (m_tramp, SImode, TARGET_32BIT ? 12 : 16);
> > +      fnaddr = XEXP (DECL_RTL (fndecl), 0);
> > +      emit_move_insn (mem, fnaddr);
> > +    }
> >
> >    a_tramp = XEXP (m_tramp, 0);
> >    emit_library_call (gen_rtx_SYMBOL_REF (Pmode, "__clear_cache"),
> > @@ -4016,7 +4078,9 @@ arm_trampoline_init (rtx m_tramp, tree fndecl, rtx chain_value)
> >  static rtx
> >  arm_trampoline_adjust_address (rtx addr)
> >  {
> > -  if (TARGET_THUMB)
> > +  /* For FDPIC don't fix trampoline address since it's a function
> > +     descriptor and not a function address.  */
> > +  if (TARGET_THUMB && !TARGET_FDPIC)
> >      addr = expand_simple_binop (Pmode, IOR, addr, const1_rtx,
> >                                  NULL, 0, OPTAB_LIB_WIDEN);
> >    return addr;
> > diff --git a/gcc/config/arm/arm.h b/gcc/config/arm/arm.h
> > index 4671d64..22a65a1 100644
> > --- a/gcc/config/arm/arm.h
> > +++ b/gcc/config/arm/arm.h
> > @@ -1581,7 +1581,7 @@ typedef struct
> >  #define INIT_EXPANDERS  arm_init_expanders ()
> >
> >  /* Length in units of the trampoline for entering a nested function.  */
> > -#define TRAMPOLINE_SIZE  (TARGET_32BIT ? 16 : 20)
> > +#define TRAMPOLINE_SIZE  (TARGET_FDPIC ? 32 : (TARGET_32BIT ? 16 : 20))
> >
> >  /* Alignment required for a trampoline in bits.  */
> >  #define TRAMPOLINE_ALIGNMENT  32
> > --
> > 2.6.3
> >
>