MIPS: Add `.insn' to ensure a text label is defined as code not data
Commit Message
Avoid a build error with microMIPS compilation and recent versions of
GAS which complain if a branch targets a label which is marked as data
rather than microMIPS code:
../sysdeps/mips/mips32/crti.S: Assembler messages:
../sysdeps/mips/mips32/crti.S:72: Error: branch to a symbol in another ISA mode
make[2]: *** [.../csu/crti.o] Error 1
as commit 9d862524f6ae ("MIPS: Verify the ISA mode and alignment of
branch and jump targets") closed a hole in branch processing, making
relocation calculation respect the ISA mode of the symbol referred.
This allowed diagnosing the situation where an attempt is made to pass
control from code assembled for one ISA mode to code assembled for a
different ISA mode and either relaxing the branch to a cross-mode jump
or if that is not possible, then reporting this as an error rather than
letting such code build and then fail unpredictably at the run time.
This however requires the correct annotation of branch targets as code,
because the ISA mode is not relevant for data symbols and is therefore
not recorded for them. The `.insn' pseudo-op is used for this purpose
and has been supported by GAS since:
Wed Feb 12 14:36:29 1997 Ian Lance Taylor <ian@cygnus.com>
* config/tc-mips.c (mips_pseudo_table): Add "insn".
(s_insn): New static function.
* doc/c-mips.texi: Document .insn.
so there has been no reason to avoid it where required. More recently
this pseudo-op has been documented, by the microMIPS architecture
specification[1][2], as required for the correct interpretation of any
code label which is not followed by an actual instruction in an assembly
source.
Use it in our crti.S files then, to mark that the trailing label there
with no instructions following is indeed not a code bug and the branch
is legitimate.
References:
[1] "MIPS Architecture for Programmers, Volume II-B: The microMIPS32
Instruction Set", MIPS Technologies, Inc., Document Number: MD00582,
Revision 5.04, January 15, 2014, Section 7.1 "Assembly-Level
Compatibility", p. 533
[2] "MIPS Architecture for Programmers, Volume II-B: The microMIPS64
Instruction Set", MIPS Technologies, Inc., Document Number: MD00594,
Revision 5.04, January 15, 2014, Section 8.1 "Assembly-Level
Compatibility", p. 623
2016-11-17 Matthew Fortune <Matthew.Fortune@imgtec.com>
Maciej W. Rozycki <macro@imgtec.com>
* sysdeps/mips/mips32/crti.S (_init): Add `.insn' pseudo-op at
`.Lno_weak_fn' label.
* sysdeps/mips/mips64/n32/crti.S (_init): Likewise.
* sysdeps/mips/mips64/n64/crti.S (_init): Likewise.
---
This should qualify as obvious, however for good measure I pushed it
through full regression testing with the `mips-mti-linux-gnu' target and
big-endian regular MIPS o32 and n64 multilibs as well as little-endian o32
MIPS16 and microMIPS multilibs, all with binutils 2.27 predating the extra
branch/jump checks (or otherwise glibc wouldn't build without this patch
for the microMIPS multilib).
While working on this change I've noticed the R_MIPS_JALR regular MIPS
relocations, used unconditionally even with microMIPS compilations where
R_MICROMIPS_JALR relocations (if any) need to be used instead. Support
for R_MICROMIPS_JALR handling is incomplete in the BFD linker meaning
these relocs are always ignored, however this may change sometime and we
ought to have correct code, so I'll send a separate change to correct it,
once it has passed regression testing with a similar set of multilibs.
Meanwhile OK to apply this change?
Maciej
glibc-umips-insn.diff
Comments
On Thu, 17 Nov 2016, Maciej W. Rozycki wrote:
> 2016-11-17 Matthew Fortune <Matthew.Fortune@imgtec.com>
> Maciej W. Rozycki <macro@imgtec.com>
>
> * sysdeps/mips/mips32/crti.S (_init): Add `.insn' pseudo-op at
> `.Lno_weak_fn' label.
> * sysdeps/mips/mips64/n32/crti.S (_init): Likewise.
> * sysdeps/mips/mips64/n64/crti.S (_init): Likewise.
OK. We may want to backport this to release branches.
On Thu, 17 Nov 2016, Joseph Myers wrote:
> > * sysdeps/mips/mips32/crti.S (_init): Add `.insn' pseudo-op at
> > `.Lno_weak_fn' label.
> > * sysdeps/mips/mips64/n32/crti.S (_init): Likewise.
> > * sysdeps/mips/mips64/n64/crti.S (_init): Likewise.
>
> OK. We may want to backport this to release branches.
Indeed and I can do it while I am at it. How far back do you think it
would make sense to go with it?
Patch committed now, thanks for your review.
Maciej
On Fri, 18 Nov 2016, Maciej W. Rozycki wrote:
> On Thu, 17 Nov 2016, Joseph Myers wrote:
>
> > > * sysdeps/mips/mips32/crti.S (_init): Add `.insn' pseudo-op at
> > > `.Lno_weak_fn' label.
> > > * sysdeps/mips/mips64/n32/crti.S (_init): Likewise.
> > > * sysdeps/mips/mips64/n64/crti.S (_init): Likewise.
> >
> > OK. We may want to backport this to release branches.
>
> Indeed and I can do it while I am at it. How far back do you think it
> would make sense to go with it?
The last couple (2.24 and 2.23) seem to be in reasonably active use (i.e.
getting patches backported to them). I don't know how likely people are
to use any older branches with current binutils.
On Fri, 18 Nov 2016, Joseph Myers wrote:
> > Indeed and I can do it while I am at it. How far back do you think it
> > would make sense to go with it?
>
> The last couple (2.24 and 2.23) seem to be in reasonably active use (i.e.
> getting patches backported to them). I don't know how likely people are
> to use any older branches with current binutils.
Me neither and myself I tend to stick to the master branch. Patch
backported now, to both versions.
Maciej
===================================================================
@@ -74,6 +74,7 @@
.reloc 1f,R_MIPS_JALR,PREINIT_FUNCTION
1: jalr $25
.Lno_weak_fn:
+ .insn
#else
lw $25,%got(PREINIT_FUNCTION)($28)
.reloc 1f,R_MIPS_JALR,PREINIT_FUNCTION
===================================================================
@@ -74,6 +74,7 @@
.reloc 1f,R_MIPS_JALR,PREINIT_FUNCTION
1: jalr $25
.Lno_weak_fn:
+ .insn
#else
lw $25,%got_disp(PREINIT_FUNCTION)($28)
.reloc 1f,R_MIPS_JALR,PREINIT_FUNCTION
===================================================================
@@ -74,6 +74,7 @@
.reloc 1f,R_MIPS_JALR,PREINIT_FUNCTION
1: jalr $25
.Lno_weak_fn:
+ .insn
#else
ld $25,%got_disp(PREINIT_FUNCTION)($28)
.reloc 1f,R_MIPS_JALR,PREINIT_FUNCTION