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* glibc build failure due to Systemtap probe change
@ 2021-11-16  9:29 Florian Weimer
  2021-11-16 21:52 ` Stan Cox
  2021-11-18 13:33 ` Stefan Liebler
  0 siblings, 2 replies; 12+ messages in thread
From: Florian Weimer @ 2021-11-16  9:29 UTC (permalink / raw)
  To: Stan Cox, Stefan Liebler; +Cc: Arjun Shankar, libc-alpha, systemtap

On Fedora rawhide, glibc no longer builds:

In file included from ../include/stap-probe.h:25,
                 from ../sysdeps/s390/s390-64/__longjmp.c:24:
../sysdeps/s390/s390-64/__longjmp.c: In function '__longjmp':
../sysdeps/s390/s390-64/__longjmp.c:52:50: error: expected ':' or ')' before numeric constant
   52 |                         LIBC_PROBE_ASM (longjmp, 8@%1 -4@%0 8@%%r4)
      |                                                  ^
../sysdeps/s390/s390-64/__longjmp.c:52:25: note: in expansion of macro 'LIBC_PROBE_ASM'
   52 |                         LIBC_PROBE_ASM (longjmp, 8@%1 -4@%0 8@%%r4)
      |                         ^~~~~~~~~~~~~~

This is because of a Systemtap probe:

| /* Jump to the position specified by ENV, causing the
|    setjmp call there to return VAL, or 1 if VAL is 0.  */
| void
| __longjmp (__jmp_buf env, int val)
| {
| #ifdef PTR_DEMANGLE
|   uintptr_t guard = THREAD_GET_POINTER_GUARD ();
| # ifdef CHECK_SP
|   CHECK_SP (env, guard);
| # endif
| #elif defined CHECK_SP
|   CHECK_SP (env, 0);
| #endif
|   register long int r2 __asm__ ("%r2") = val == 0 ? 1 : val;
| #ifdef PTR_DEMANGLE
|   register uintptr_t r3 __asm__ ("%r3") = guard;
|   register void *r1 __asm__ ("%r1") = (void *) env;
| #endif
|   /* Restore registers and jump back.  */
|   __asm__ __volatile__ (
| 			/* longjmp probe expects longjmp first argument, second
| 			   argument and target address.  */
| #ifdef PTR_DEMANGLE
| 			"lmg  %%r4,%%r5,64(%1)\n\t"
| 			"xgr  %%r4,%2\n\t"
| 			"xgr  %%r5,%2\n\t"
| 			LIBC_PROBE_ASM (longjmp, 8@%1 -4@%0 8@%%r4)
| #else
| 			LIBC_PROBE_ASM (longjmp, 8@%1 -4@%0 8@%%r14)
| #endif

My guess this is related to the macro changes to support floating point
values in probes added in this commit:

commit eaa15b047688175a94e3ae796529785a3a0af208
Author: Stan Cox <scox@redhat.com>
Date:   Thu Sep 30 16:11:29 2021 -0400

    PR27829: Support floating point values passed through sdt.h markers
    
    Add the type to the individual arg entries in the .notes.stapsdt section;
    currently SP@A, where S is optional '-' sign, P is precision of type and A is
    address.  Revised format is SPT@A where T is optional 'f' for float variables.
    Add x8664 float registers xmm8 - xmm15 and aarch64 float registers v8 - v31.
    Parse the type field; result is currently ignored.  asm statements are
    restricted to 30 arguments; sdt probes can have up to 12 arguments.  To fit
    this into a single asm statement, precision and type are encoded into a single
    field: 0xSSTT where SS is the precision and TT is the type as encoded by
    __builtin_classify_type.  The sign S, precision P, and type T are decoded by
    _SDT_SIGN, _SDT_SIZE, and _SDT_TYPE.  Test that the revised
    .notes.stapsdt section interacts correctly with eu-elfutils and gdb.

The macro changes in this commit seem to trigger additional macro
expansion in the preprocessor, and therefore require syntax closer to C.

LIBC_PROBE_ASM was added to longjmp to support GDB, but if GDB really
needs this, I'm not sure if Systemtap probes (via <sys/sdt.h>) are the
right mechanism because building with Systemtap support is optional.
Maybe we should copy <sys/sdt.h> into the glibc sources, and then we
could drop the floating point support again. 8-/

Or we could rewrite s390/s390x __longjmp in assembler.  It's mostly
assembler anyway.  Stefan, what do you think?

Thanks,
Florian


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