[PATCH 1/5] string: Add fallback implementation for ctz/clz

[Adhemerval Zanella Netto]

On 01/09/25 14:09, Cristian Rodríguez wrote:
> On Mon, Sep 1, 2025 at 11:40 AM Adhemerval Zanella Netto
> <adhemerval.zanella@linaro.org> wrote:
> 
>> I found on s390x that calling the libgcc builtin is not fully supported without
>> setting up TLS because the builtin could be built with stack-protection enabled.
>> It might happen on other ABIs that uses the generic strchrnul implementation and
>> relying on libgcc for __builtin_c[t,l]z.
> 
> Huh..what? everything else I have seen assumes this builtins are unsequenced..
> is this hiding a bug somewhere?

I am not sure, but using the tst-assert-startup-static test from patchset:

$ qemu-s390x -g 1234 elf/tst-assert-startup-static
[...]

$ gdb-multiarch
[...]
(gdb) target remote localhost:1234
[...]
_start () at ../sysdeps/s390/s390-64/start.S:65
65		la	%r4,8(%r15)		# get argv
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x000002aa000972b6 in __clzdi2 (x=<optimized out>) at /home/azanella/toolchain/src/gcc/libgcc/libgcc2.c:692
692	  return ret;
(gdb) bt
#0  0x000002aa000972b6 in __clzdi2 (x=<optimized out>) at /home/azanella/toolchain/src/gcc/libgcc/libgcc2.c:692
#1  0x000002aa00014f2a in clz (c=<optimized out>) at ../sysdeps/generic/string-fzi.h:60
#2  index_first (c=<optimized out>) at ../sysdeps/generic/string-fzi.h:115
#3  index_first_zero_eq (x1=<optimized out>, x2=<optimized out>) at ../sysdeps/generic/string-fzc.h:59
#4  __strchrnul_c (str=<optimized out>, c_in=<optimized out>) at ../string/strchrnul.c:51
#5  0x000002aa0000a0e6 in __libc_message_impl (fmt=fmt@entry=0x2aa000a1eee "Fatal glibc error: %s:%s (%s): assertion failed: %s\n")
    at ../sysdeps/posix/libc_fatal.c:77
#6  0x000002aa00009338 in __libc_assert_fail (assertion=assertion@entry=0x2aa000a5ef6 "error",
    file=file@entry=0x2aa000a1d80 "tst-assert-startup-static.c", line=line@entry=28,
    function=function@entry=0x2aa000a1d9c <__func__.0> "__tunables_init") at __libc_assert_fail.c:31
#7  0x000002aa0000820e in __tunables_init (env=<optimized out>) at tst-assert-startup-static.c:28
#8  0x000002aa00008ab8 in __libc_start_main_impl (main=0x2aa00008104 <__wrap_main>, argc=1, argv=0x200007ffb38, init=<optimized out>,
    fini=<optimized out>, rtld_fini=0x0, stack_end=0x200007ffa80) at ../csu/libc-start.c:267
#9  0x000002aa00008102 in _start () at ../sysdeps/s390/s390-64/start.S:101
(gdb) disas
Dump of assembler code for function __clzdi2:
   0x000002aa00097260 <+0>:	lghi	%r1,56
   0x000002aa00097264 <+4>:	lghi	%r3,7
   0x000002aa00097268 <+8>:	srlg	%r4,%r2,0(%r1)
   0x000002aa0009726e <+14>:	tmll	%r4,255
   0x000002aa00097272 <+18>:	jne	0x2aa0009729e <__clzdi2+62>
   0x000002aa00097276 <+22>:	aghi	%r1,-8
   0x000002aa0009727a <+26>:	brctg	%r3,0x2aa00097268 <__clzdi2+8>
   0x000002aa0009727e <+30>:	larl	%r5,0x2aa000d0ed8
   0x000002aa00097284 <+36>:	lghi	%r0,64
   0x000002aa00097288 <+40>:	lg	%r1,0(%r5)
   0x000002aa0009728e <+46>:	llgc	%r2,0(%r2,%r1)
   0x000002aa00097294 <+52>:	sgr	%r0,%r2
   0x000002aa00097298 <+56>:	lgfr	%r2,%r0
   0x000002aa0009729c <+60>:	br	%r14
   0x000002aa0009729e <+62>:	lgr	%r2,%r4
   0x000002aa000972a2 <+66>:	larl	%r5,0x2aa000d0ed8
   0x000002aa000972a8 <+72>:	lghi	%r0,64
   0x000002aa000972ac <+76>:	sgr	%r0,%r1
   0x000002aa000972b0 <+80>:	lg	%r1,0(%r5)
=> 0x000002aa000972b6 <+86>:	llgc	%r2,0(%r2,%r1)
   0x000002aa000972bc <+92>:	sgr	%r0,%r2
   0x000002aa000972c0 <+96>:	lgfr	%r2,%r0
   0x000002aa000972c4 <+100>:	br	%r14
End of assembler dump.
(gdb) i r r2 r1
r2             0x80                128
r1             0xbe148             778568
(gdb) p &__clz_tab
$1 = (const UQItype (*)[256]) 0x2aa000be148 <__clz_tab>

Digging into further, it seems that for s390 we need to self-relocate it first
before being able to access __clz_tab.