--fp-type causes SIGILL with i386 Linux and old Clang

[infrastation]

Whilst testing various things on an old Pentium 3 PC, I noticed that Clang build of tcpdump cannot be tested. The OS is Debian 6.0.10. The kernel is supported, at least on paper. If this problem is not worth fixing, it would be useful at least to document it. Remote access can be provided if necessary.

kernel 1: Linux debian6-32bit 2.6.32-5-486 #1 Tue May 13 16:15:32 UTC 2014 i686 GNU/Linux
kernel 2: Linux debian6-32bit 2.6.32-5-686 #1 SMP Tue May 13 16:33:32 UTC 2014 i686 GNU/Linux

$ gcc --version
gcc (Debian 4.4.5-8) 4.4.5
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ make check
./tests/TESTrun
./tcpdump --fp-type => 2
./tcpdump --time-t-size => 32
[...]

$ clang --version
clang version 1.1 (Debian 2.7-3)
Target: i386-pc-linux-gnu
Thread model: posix

$ make check
./tests/TESTrun
ERROR: piping tcpdump --fp-type failed at close
make: *** [check] Error 255

$ ./tcpdump --fp-type
Illegal instruction

# try with GDB
Program received signal SIGILL, Illegal instruction.
0x0804a40b in float_type_check () at fptype.c:49
49		printf("%.3f\n", f.f*8/1000000);
(gdb) bt
#0  0x0804a40b in float_type_check () at fptype.c:49
#1  0x0804b219 in main () at tcpdump.c:2116

[guyharris]

Whilst testing various things on an old Pentium 3 PC

...which, at least as I read the Pentium 3 Wikipedia page, has SSE but not SSE2 or later.

Sadly, not all platforms 1) support executable files being tagged not only with an instruction set but a version of an instruction set, e.g. tagging a program as "needs at least this version of 32-bit x86", 2) have an "your processor cannot execute that code" error and return it for programs that require instruction set features that the machine doesn't support, and 3) report that error to the user.

I noticed that Clang build of tcpdump cannot be tested

Clang might default to building for 32-bit x86 with SSE2; if so ,you might have to use some -m flag to tell it to generate code that doesn't use SSE2.

--fp-type causes SIGILL

So might dissecting packets that contain floating-point numbers.

[infrastation]

The problem does not reproduce using the latest Debian 12 on the same hardware:

# uname -a
Linux debian12-32bit 6.1.0-37-686-pae #1 SMP PREEMPT_DYNAMIC Debian 6.1.140-1 (2025-05-22) i686 GNU/Linux

# lscpu 
Architecture:                i686
  CPU op-mode(s):            32-bit
  Address sizes:             36 bits physical, 32 bits virtual
  Byte Order:                Little Endian
CPU(s):                      1
  On-line CPU(s) list:       0
Vendor ID:                   GenuineIntel
  BIOS Vendor ID:            Intel
  Model name:                Pentium III (Katmai)
    BIOS Model name:           CPU @ 0.5GHz
    BIOS CPU family:         17
    CPU family:              6
    Model:                   7
    Thread(s) per core:      1
    Core(s) per socket:      1
    Socket(s):               1
    Stepping:                3
    BogoMIPS:                1097.25
    Flags:                   fpu vme de pse tsc msr pae mce cx8 sep mtrr pge mca cmov pse36 mmx fxsr sse cpuid pti
Vulnerabilities:             
  Gather data sampling:      Not affected
  Indirect target selection: Not affected
  Itlb multihit:             KVM: Mitigation: VMX unsupported
  L1tf:                      Mitigation; PTE Inversion
  Mds:                       Vulnerable: Clear CPU buffers attempted, no microcode; SMT disabled
  Meltdown:                  Mitigation; PTI
  Mmio stale data:           Unknown: No mitigations
  Reg file data sampling:    Not affected
  Retbleed:                  Not affected
  Spec rstack overflow:      Not affected
  Spec store bypass:         Vulnerable
  Spectre v1:                Mitigation; usercopy/swapgs barriers and __user pointer sanitization
  Spectre v2:                Mitigation; Retpolines; STIBP disabled; RSB filling; PBRSB-eIBRS Not affected; BHI Not affected
  Srbds:                     Not affected
  Tsx async abort:           Not affected

$ clang --version
Debian clang version 14.0.6
Target: i386-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

$ ./tcpdump --fp-type
9877.894

Whether the root cause is a particular version of a particular compiler or something else, it may be a good idea to have a build-time check that would attempt the same as what float_type_check() does and would either activate a workaround or at least fail the build reliably.