linux-kernel - Re: [PATCH 2/2] recordmcount: Handle sections with no non-weak symbols

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Message-ID: <72d17597-874c-f3a3-9398-0cc944350c5b@csgroup.eu>
Date:   Wed, 4 May 2022 16:50:58 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 2/2] recordmcount: Handle sections with no non-weak
 symbols



Le 03/05/2022 à 18:25, Steven Rostedt a écrit :
> On Tue, 3 May 2022 11:20:22 +0000
> Christophe Leroy <christophe.leroy@...roup.eu> wrote:
> 
>> Maybe I misunderstood. When you say 'after linking', do you mean vmlinux
>> or vmlinux.o ?
> 
> Whichever ;-)
> 
>>
>> In vmlinux, the addresses to be saved in __mcount_loc table might not
>> contain anymore a call to _mcount but a call to a trampoline that jumps
>> to _mcount, in case _mcount is too far from the said location at link
>> time. That's what I meant.
> 
> But how is that different than what is done today? And at linking,
> everything still calls mcount. It's not until runtime things change.

Everything call _mcount but not directly.

In vmlinux, relocations are resolved, trampolines are installed for 
unreachable destinations and you don't anymore have a section with all 
the relocations to mcount. It means 'recordmcount' or whatever tool we 
use will have to read the code to find all direct calls to mcount, then 
find all trampolines to mcount then find all calls to those trampolines.

See below some code extracted from vmlinux on a allyesconfig powerpc64le 
build:

c000000000012300 <__traceiter_initcall_level>:
c000000000012300:       4c 1a 4c 3c     addis   r2,r12,6732
c000000000012304:       00 be 42 38     addi    r2,r2,-16896
c000000000012308:       a6 02 08 7c     mflr    r0
c00000000001230c:       4d 60 0d 48     bl      c0000000000e8358 <_mcount>
c000000000012310:       a6 02 08 7c     mflr    r0
...
c0000000020e8740 <get_cur_path>:
c0000000020e8740:       3e 18 4c 3c     addis   r2,r12,6206
c0000000020e8744:       c0 59 42 38     addi    r2,r2,22976
c0000000020e8748:       a6 02 08 7c     mflr    r0
c0000000020e874c:       c5 ff 7c 48     bl      c0000000028b8710 
<0003af2b.plt_branch._mcount>
c0000000020e8750:       a6 02 08 7c     mflr    r0
...
c0000000028b8710 <0003af2b.plt_branch._mcount>:
c0000000028b8710:       ff ff 82 3d     addis   r12,r2,-1
c0000000028b8714:       98 8e 8c e9     ld      r12,-29032(r12)
c0000000028b8718:       a6 03 89 7d     mtctr   r12
c0000000028b871c:       20 04 80 4e     bctr
...
c0000000044bdcc0 <handle_lcd_irq.isra.0>:
c0000000044bdcc0:       01 16 4c 3c     addis   r2,r12,5633
c0000000044bdcc4:       40 04 42 38     addi    r2,r2,1088
c0000000044bdcc8:       a6 02 08 7c     mflr    r0
c0000000044bdccc:       fd 2d c0 49     bl      c0000000060c0ac8 
<000a751f.plt_branch._mcount>
c0000000044bdcd0:       a6 02 08 7c     mflr    r0
...
c0000000060c0ac8 <000a751f.plt_branch._mcount>:
c0000000060c0ac8:       ff ff 82 3d     addis   r12,r2,-1
c0000000060c0acc:       98 8e 8c e9     ld      r12,-29032(r12)
c0000000060c0ad0:       a6 03 89 7d     mtctr   r12
c0000000060c0ad4:       20 04 80 4e     bctr
...


Christophe