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Message-ID:
<SYBP282MB02029BB58835B4909F7D04938AB69@SYBP282MB0202.AUSP282.PROD.OUTLOOK.COM>
Date: Mon, 1 Feb 2021 00:44:49 +0000
From: Jiahao XU <Jiahao_XU@...look.com>
To: Rich Felker <dalias@...c.org>
CC: "musl@...ts.openwall.com" <musl@...ts.openwall.com>
Subject: Re: Can’t build musl with lto=thin
Correction: The executable is cut from 48KB to 8KB.
I mixed up the numbers from du and ls.
Jiahao XU
________________________________
From: Jiahao XU <Jiahao_XU@...look.com>
Sent: Monday, February 1, 2021 11:31:47 AM
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com <musl@...ts.openwall.com>
Subject: Re: [musl] Can’t build musl with lto=thin
Interesting enough, I found —gc-section used along with -flto can cut the size of final hello_world executable from 48KB to 5KB.
After investigating with bloaty, I found that —gc-section along with -flto is able to cut .text from 25.4 KiB to 3.04 KiB, and cut the .rodata from 19.5 KiB to 120 bytes.
.data section however, seen an increase from 316 bytes to 372 bytes, but the VM size is cut from 252 to 244 bytes.
$ bloaty gc-section-a.out -- no-gc-section.a.out
FILE SIZE VM SIZE
-------------- --------------
+18% +56 -3.2% -8 .data
[NEW] +6 [NEW] +6 [LOAD #2 [RX]]
[DEL] -4 -66.7% -8 [LOAD #4 [RW]]
-72.7% -8 [ = ] 0 [Unmapped]
-32.0% -64 [ = ] 0 .comment
-99.4% -19.4Ki -99.7% -19.4Ki .rodata
-88.0% -22.3Ki -88.2% -22.3Ki .text
-89.4% -41.8Ki -88.5% -41.8Ki TOTAL
Jiahao XU
________________________________
From: Jiahao XU <Jiahao_XU@...look.com>
Sent: Monday, February 1, 2021 11:22:21 AM
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com <musl@...ts.openwall.com>
Subject: Re: [musl] Can’t build musl with lto=thin
I have finally succeded to produce a statically linked hello_world executable.
I modified the last line of musl-clang-lld to:
exec $($cc -print-prog-name=ld.lld) -nostdlib “$@“ -l:libc.a —no-dynamic-linker
and everything works fine now.
Jiahao XU
________________________________
From: Rich Felker <dalias@...c.org>
Sent: Monday, February 1, 2021 8:01:06 AM
To: Jiahao XU <Jiahao_XU@...look.com>
Cc: musl@...ts.openwall.com <musl@...ts.openwall.com>
Subject: Re: [musl] Can’t build musl with lto=thin
On Sun, Jan 31, 2021 at 05:32:45AM +0000, Jiahao XU wrote:
> I used `musl-clang -Oz -flto -s -fuse-ld=musl-clang-lld-static -Wl,—plugin-opt=O3,-O3 hello.c` to produce the executable.
Where is -static? Normally it does *not* work to add -static just to
the ld command line. The compiler driver has to know that it's
requesting static linking because it will pass a different command
line to the linker based on that.
> Content of `/usr/local/musl/bin/ld.musl-clang-lld-static` is same as
> the generated `ld.musl-clang`, except for the last line, which I
> modified it to:
>
> exec $($cc -print-prog-name=ld.lld) -nostdlib “$@“ -static -lc -dynamic-linker “$ldso”
Try moving -static out from here (i.e. using the script unmodified
except for requesting ld.lld) and see if that works. Note that a
correctly linked executable will not have any INTERP in readelf -a
output, so as long as you see INTERP anywhere there you're doing
something wrong.
Rich
> ________________________________
> From: Rich Felker <dalias@...c.org>
> Sent: Sunday, January 31, 2021 4:01:22 PM
> To: Jiahao XU <Jiahao_XU@...look.com>
> Cc: musl@...ts.openwall.com <musl@...ts.openwall.com>
> Subject: Re: [musl] Can’t build musl with lto=thin
>
> On Sat, Jan 30, 2021 at 11:44:48PM +0000, Jiahao XU wrote:
> > (gdb) bt
> >
> > #0 0x00007ffff7ff5498 in decode_vec () from /lib/ld-musl-x86_64.so.1
> >
> > #1 0x00007ffff7ff58cb in decode_dyn () from /lib/ld-musl-x86_64.so.1
> >
> > #2 0x0000000000000000 in ?? ()
>
> This is not a static-linked program. decode_dyn is part of the dynamic
> linker. It looks to me like you've created some sort of weird hybrid
> executable that's not valid. Can you show the command lines you used
> to produce it?
>
> Also please keep list on CC when replying.
>
> Rich
>
>
> > (gdb) info r
> >
> > rax 0x1 1
> >
> > rbx 0x7ffff7ffe2d8 140737354130136
> >
> > rcx 0x5000 20480
> >
> > rdx 0x200238 2097720
> >
> > rsi 0x7fffffffd8d0 140737488345296
> >
> > rdi 0x0 0
> >
> > rbp 0x7ffff7ffe2d8 0x7ffff7ffe2d8 <__dls3.app>
> >
> > rsp 0x7fffffffd8c8 0x7fffffffd8c8
> >
> > r8 0x0 0
> >
> > r9 0xfffffffffffff000 -4096
> >
> > r10 0x800000 8388608
> >
> > r11 0x200000 2097152
> >
> > r12 0x7fffffffdcb8 140737488346296
> >
> > r13 0x0 0
> >
> > r14 0x7fffffffd8d0 140737488345296
> >
> > r15 0x7fffffffdcb8 140737488346296
> >
> > rip 0x7ffff7ff5498 0x7ffff7ff5498 <decode_vec+21>
> >
> > eflags 0x10246 [ PF ZF IF RF ]
> >
> > cs 0x33 51
> >
> > ss 0x2b 43
> >
> > ds 0x0 0
> >
> > es 0x0 0
> >
> > fs 0x0 0
> >
> > gs 0x0 0
> >
> >
> > Disassembly of decode_dyn:
> >
> >
> > 0x00007ffff7ff58af <+0>: push %r14
> >
> > 0x00007ffff7ff58b1 <+2>: push %rbx
> >
> > 0x00007ffff7ff58b2 <+3>: sub $0x108,%rsp
> >
> > 0x00007ffff7ff58b9 <+10>: mov %rdi,%rbx
> >
> > 0x00007ffff7ff58bc <+13>: mov 0x10(%rdi),%rdi
> >
> > 0x00007ffff7ff58c0 <+17>: mov %rsp,%r14
> >
> > 0x00007ffff7ff58c3 <+20>: mov %r14,%rsi
> >
> > 0x00007ffff7ff58c6 <+23>: call 0x7ffff7ff5483 <decode_vec>
> >
> > 0x00007ffff7ff58cb <+28>: mov (%rbx),%rax
> >
> >
> > Disassembly of decode_vec:
> >
> >
> > 0x00007ffff7ff5483 <+0>: xor %eax,%eax
> >
> > 0x00007ffff7ff5485 <+2>: cmp $0x20,%rax
> >
> > 0x00007ffff7ff5489 <+6>: je 0x7ffff7ff5495 <decode_vec+18>
> >
> > 0x00007ffff7ff548b <+8>: andq $0x0,(%rsi,%rax,8)
> >
> > 0x00007ffff7ff5490 <+13>: inc %rax
> >
> > 0x00007ffff7ff5493 <+16>: jmp 0x7ffff7ff5485 <decode_vec+2>
> >
> > 0x00007ffff7ff5495 <+18>: push $0x1
> >
> > 0x00007ffff7ff5497 <+20>: pop %rax
> >
> > => 0x00007ffff7ff5498 <+21>: mov (%rdi),%rcx
> >
> > 0x00007ffff7ff549b <+24>: test %rcx,%rcx
> >
> > 0x00007ffff7ff549e <+27>: je 0x7ffff7ff54c3 <decode_vec+64>
> >
> > 0x00007ffff7ff54a0 <+29>: lea -0x1(%rcx),%rdx
> >
> > 0x00007ffff7ff54a4 <+33>: cmp $0x1e,%rdx
> >
> > 0x00007ffff7ff54a8 <+37>: ja 0x7ffff7ff54bd <decode_vec+58>
> >
> > 0x00007ffff7ff54aa <+39>: shlx %rcx,%rax,%rcx
> >
> > 0x00007ffff7ff54af <+44>: or %rcx,(%rsi)
> >
> > 0x00007ffff7ff54b2 <+47>: mov (%rdi),%rcx
> >
> > 0x00007ffff7ff54b5 <+50>: mov 0x8(%rdi),%rdx
> >
> > 0x00007ffff7ff54b9 <+54>: mov %rdx,(%rsi,%rcx,8)
> >
> > 0x00007ffff7ff54bd <+58>: add $0x10,%rdi
> >
> > 0x00007ffff7ff54c1 <+62>: jmp 0x7ffff7ff5498 <decode_vec+21>
> >
> > 0x00007ffff7ff54c3 <+64>: ret
> >
> >
> > Jiahao XU
> >
> > Get Outlook for iOS<https://aka.ms/o0ukef>
> > ________________________________
> > From: Rich Felker <dalias@...c.org>
> > Sent: Sunday, January 31, 2021 10:30:12 AM
> > To: Jiahao XU <Jiahao_XU@...look.com>
> > Cc: musl@...ts.openwall.com <musl@...ts.openwall.com>
> > Subject: Re: [musl] Can’t build musl with lto=thin
> >
> > On Sat, Jan 30, 2021 at 11:04:32PM +0000, Jiahao XU wrote:
> > > > So something like (in config.mak):
> > > >
> > > > obj/ldso/dlstart.lo: CFLAGS_ALL += -fno-lto
> > >
> > > Thanks, with this I was able to build libc.so successfully with clang and created a 3.5 KB hello world program using clang and lld.
> > >
> > > However, I still wasn’t able to statically linked with libc.
> > >
> > > Once I added ‘-static’ to the compiler flags, the executable failed with ‘Segmentation fault (core dumped)’.
> >
> > It's libc.a, not libc.so, that will be involved in making a
> > static-linked binary. It's hard to know what's going wrong without
> > more information. Can you run under a debugger and provide a
> > backtrace, disassembly, and register dump for where the crash occurs?
> >
> > Rich
> >
> >
> > > ________________________________
> > > From: Rich Felker <dalias@...c.org>
> > > Sent: Sunday, January 31, 2021 7:12:31 AM
> > > To: Jiahao XU <Jiahao_XU@...look.com>
> > > Cc: musl@...ts.openwall.com <musl@...ts.openwall.com>
> > > Subject: Re: [musl] Can’t build musl with lto=thin
> > >
> > > On Fri, Jan 29, 2021 at 12:19:42PM +0000, Jiahao XU wrote:
> > > > musl-1.2.2 compilation with clang-11 failed to build libc.so at the final linking stage:
> > > >
> > > > ld.lld: error: undefined hidden symbol: __dls2
> > > > >>> referenced by ld-temp.o
> > > > >>> lto.tmp:(_dlstart_c)
> > > > >>> did you mean: __dls3
> > > > >>> defined in: lto.tmp
> > > >
> > > > I am using CFLAGS=‘-march=native -mtune=native -Oz -flto
> > > > -fmerge-all-constants -fomit-frame-pointer’ and LDFLAGS=‘-flto
> > > ^^^^^^^^^^^^^^^^^^^^^
> > >
> > > The -fmerge-all-constants option gives non-conforming language
> > > semantics and should not be used, but that's a separate issue.
> > >
> > > > -fuse-ld=lld -Wl,—plugin-opt=O3,-O3,—icf=safe’.
> > >
> > > > No configure option is supplied.
> > >
> > > Otherwise, it's a known issue that LTO misses references from asm
> > > (both top-level and in functions). I think dlstart.lo and a few other
> > > files should just be built with LTO disabled; any LTO-type
> > > optimization in code that runs at this stage is inherently invalid,
> > > anyway. So something like (in config.mak):
> > >
> > > obj/ldso/dlstart.lo: CFLAGS_ALL += -fno-lto
> > >
> > > Rich
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