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Re: [PATCH] x86-64: Resolve R_X86_64_PLT32 referencing a local symbol even if defined in another section
On Thu, Feb 13, 2020 at 11:29 AM Fangrui Song <maskray@google.com> wrote:
>
> On 2020-02-13, H.J. Lu wrote:
> >On Thu, Feb 13, 2020 at 10:05 AM Fangrui Song <maskray@google.com> wrote:
> >>
> >>
> >> On 2020-02-13, H.J. Lu wrote:
> >> >On Thu, Feb 13, 2020 at 9:29 AM Fangrui Song <maskray@google.com> wrote:
> >> >>
> >> >> On 2020-02-13, H.J. Lu wrote:
> >> >> >On Wed, Feb 12, 2020 at 11:25 PM Fangrui Song <maskray@google.com> wrote:
> >> >> >>
> >> >> >> See gas/testsuite/gas/i386/relax-5.s
> >> >> >> Currently gas incorrectly emits a .L symbol.
> >> >> >> Context: https://github.com/ClangBuiltLinux/linux/issues/811
> >>
> >> Sorry. I pasted a wrong link. It should be
> >> https://github.com/ClangBuiltLinux/linux/issues/872
> >>
> >> >> >> (begin story
> >> >> >>
> >> >> >> I taught clang to emit a call insn referencing a local symbol if the
> >> >> >> symbol is defined in the same translation unit. This can avoid unneeded
> >> >> >> PLT if the object file is linked with -shared.
> >> >> >
> >> >> >Why does a linker generate a PLT entry for PLT32 relocation against a local
> >> >> >symbol? Does BFD linker have the same issue?
> >> >>
> >> >> The problem is not a linker, but objtool (used by the Linux kernel)'s
> >> >> unneeded constraint on st_type.
> >> >>
> >> >
> >> >I don't have any problem with PLT32 and objtool in the current Linux kernel.
> >> >Are you using very old Linux kernel?
> >>
> >> This is related to the latest change in linux-next.
> >>
> >>
> >> .section .init.text,"ax",@progbits
> >> call .Lprintk$local
> >> # -- End function
> >> .text
> >> .globl printk # -- Begin function printk
> >> .type printk,@function
> >> printk: # @printk
> >> .Lprintk$local:
> >> ret
> >>
> >> % as a.s -o a.o
> >> % readelf -Wrs a.o
> >>
> >> Relocation section '.rela.init.text' at offset 0x108 contains 1 entry:
> >> Offset Info Type Symbol's Value Symbol's Name + Addend
> >> 0000000000000001 0000000500000004 R_X86_64_PLT32 0000000000000000 .Lprintk$local - 4
> >> ...
> >> 5: 0000000000000000 0 NOTYPE LOCAL DEFAULT 1 .Lprintk$local
> >>
> >>
> >> `call .Lprintk$local` is in a different section. It causes GNU as to
> >> emit an R_X86_64_PLT32 referencing a local symbol.
> >>
> >> From https://sourceware.org/binutils/docs/as/Symbol-Names.html
> >>
> >> > Local symbols are defined and used within the assembler, but they are normally not saved in object files.
> >>
> >> R_X86_64_PC32 does not have the problem. I think this can be seen as a
> >> missed optimization.
> >
> >Please open a binutils bug and I will take a look.
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=25551
This is the patch I am checking in.
Thanks.
--
H.J.
From 07140a481b1837792a0e896a0b1357c2168c21e2 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Thu, 13 Feb 2020 13:26:22 -0800
Subject: [PATCH] x86: Resolve PLT32 reloc aganst local symbol to section
Since PLT entry isn't needed for branch to local symbol, we can resolve
R_386_PLT32/R_X86_64_PLT32 relocation aganst local symbol to section,
similar to R_386_PC32/R_X86_64_PC32.
2020-02-13 Fangrui Song <maskray@google.com>
H.J. Lu <hongjiu.lu@intel.com>
PR gas/25551
* config/tc-i386.c (tc_i386_fix_adjustable): Don't check
BFD_RELOC_386_PLT32 nor BFD_RELOC_X86_64_PLT32.
* testsuite/gas/i386/i386.exp: Run relax-5 and x86-64-relax-4.
* testsuite/gas/i386/relax-5.d: New file.
* testsuite/gas/i386/relax-5.s: Likewise.
* testsuite/gas/i386/x86-64-relax-4.d: Likewise.
* testsuite/gas/i386/x86-64-relax-4.s: Likewise.
---
gas/config/tc-i386.c | 2 --
gas/testsuite/gas/i386/i386.exp | 2 ++
gas/testsuite/gas/i386/relax-5.d | 15 +++++++++++++++
gas/testsuite/gas/i386/relax-5.s | 8 ++++++++
gas/testsuite/gas/i386/x86-64-relax-4.d | 17 +++++++++++++++++
gas/testsuite/gas/i386/x86-64-relax-4.s | 8 ++++++++
6 files changed, 50 insertions(+), 2 deletions(-)
create mode 100644 gas/testsuite/gas/i386/relax-5.d
create mode 100644 gas/testsuite/gas/i386/relax-5.s
create mode 100644 gas/testsuite/gas/i386/x86-64-relax-4.d
create mode 100644 gas/testsuite/gas/i386/x86-64-relax-4.s
diff --git a/gas/config/tc-i386.c b/gas/config/tc-i386.c
index 6b923ccb81..8e422fd2dc 100644
--- a/gas/config/tc-i386.c
+++ b/gas/config/tc-i386.c
@@ -3398,7 +3398,6 @@ tc_i386_fix_adjustable (fixS *fixP ATTRIBUTE_UNUSED)
if (fixP->fx_r_type == BFD_RELOC_SIZE32
|| fixP->fx_r_type == BFD_RELOC_SIZE64
|| fixP->fx_r_type == BFD_RELOC_386_GOTOFF
- || fixP->fx_r_type == BFD_RELOC_386_PLT32
|| fixP->fx_r_type == BFD_RELOC_386_GOT32
|| fixP->fx_r_type == BFD_RELOC_386_GOT32X
|| fixP->fx_r_type == BFD_RELOC_386_TLS_GD
@@ -3411,7 +3410,6 @@ tc_i386_fix_adjustable (fixS *fixP ATTRIBUTE_UNUSED)
|| fixP->fx_r_type == BFD_RELOC_386_TLS_LE
|| fixP->fx_r_type == BFD_RELOC_386_TLS_GOTDESC
|| fixP->fx_r_type == BFD_RELOC_386_TLS_DESC_CALL
- || fixP->fx_r_type == BFD_RELOC_X86_64_PLT32
|| fixP->fx_r_type == BFD_RELOC_X86_64_GOT32
|| fixP->fx_r_type == BFD_RELOC_X86_64_GOTPCREL
|| fixP->fx_r_type == BFD_RELOC_X86_64_GOTPCRELX
diff --git a/gas/testsuite/gas/i386/i386.exp b/gas/testsuite/gas/i386/i386.exp
index 2ca8a94132..8cace3dc9f 100644
--- a/gas/testsuite/gas/i386/i386.exp
+++ b/gas/testsuite/gas/i386/i386.exp
@@ -580,6 +580,7 @@ if [expr ([istarget "i*86-*-*"] || [istarget "x86_64-*-*"]) && [gas_32_check]]
run_dump_test "relax-3"
run_dump_test "relax-4"
+ run_dump_test "relax-5"
run_dump_test "got"
run_dump_test "got-no-relax"
@@ -1135,6 +1136,7 @@ if [expr ([istarget "i*86-*-*"] || [istarget "x86_64-*-*"]) && [gas_64_check]] t
run_dump_test "x86-64-relax-2"
run_dump_test "x86-64-relax-3"
+ run_dump_test "x86-64-relax-4"
run_dump_test "x86-64-jump"
run_dump_test "x86-64-branch-2"
diff --git a/gas/testsuite/gas/i386/relax-5.d b/gas/testsuite/gas/i386/relax-5.d
new file mode 100644
index 0000000000..bccfe681a0
--- /dev/null
+++ b/gas/testsuite/gas/i386/relax-5.d
@@ -0,0 +1,15 @@
+#objdump: -dwr
+
+.*: +file format .*
+
+Disassembly of section .text:
+
+0+ <printk>:
+ +[a-f0-9]+: c3 ret
+
+Disassembly of section .init.text:
+
+0+ <foo>:
+ +[a-f0-9]+: e8 fb ff ff ff call 0 <foo> 1: R_386_PLT32 .text
+ +[a-f0-9]+: e8 fc ff ff ff call 6 <foo\+0x6> 6: R_386_PC32 .text
+#pass
diff --git a/gas/testsuite/gas/i386/relax-5.s b/gas/testsuite/gas/i386/relax-5.s
new file mode 100644
index 0000000000..35d5769f4a
--- /dev/null
+++ b/gas/testsuite/gas/i386/relax-5.s
@@ -0,0 +1,8 @@
+ .section .init.text,"ax",@progbits
+ .global foo
+foo:
+ call printk@PLT
+ call printk
+ .text
+printk:
+ ret
diff --git a/gas/testsuite/gas/i386/x86-64-relax-4.d b/gas/testsuite/gas/i386/x86-64-relax-4.d
new file mode 100644
index 0000000000..234e16534f
--- /dev/null
+++ b/gas/testsuite/gas/i386/x86-64-relax-4.d
@@ -0,0 +1,17 @@
+#objdump: -drw
+#notarget: *-*-solaris*
+
+.*: +file format .*
+
+
+Disassembly of section .text:
+
+0+ <printk>:
+ +[a-f0-9]+: c3 retq
+
+Disassembly of section .init.text:
+
+0+ <foo>:
+ +[a-f0-9]+: e8 00 00 00 00 callq 5 <foo\+0x5> 1: R_X86_64_PLT32 .text-0x4
+ +[a-f0-9]+: 48 8d 05 00 00 00 00 lea 0x0\(%rip\),%rax # c <foo\+0xc> 8: R_X86_64_PC32 .text-0x4
+#pass
diff --git a/gas/testsuite/gas/i386/x86-64-relax-4.s b/gas/testsuite/gas/i386/x86-64-relax-4.s
new file mode 100644
index 0000000000..683d3e2b9b
--- /dev/null
+++ b/gas/testsuite/gas/i386/x86-64-relax-4.s
@@ -0,0 +1,8 @@
+ .section .init.text,"ax",@progbits
+ .global foo
+foo:
+ call printk
+ lea printk(%rip), %rax
+ .text
+printk:
+ ret
--
2.24.1