Date: Mon, 16 May 2022 21:44:28 +0200 From: Paul Floyd <paulf2718@gmail.com> To: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: Changes to stat ABI Message-ID: <3e009edb-727e-3160-2d67-500a263ece55@gmail.com> In-Reply-To: <CANCZdfpshqwM_XvE6BUDhi48gwe2475PZG_9ZxGT2yu_611=RQ@mail.gmail.com> References: <cd9ef009-47aa-acc7-509e-7afca9b24a84@gmail.com> <CANCZdfqtS5_bzj-4Q0ERJ=qar==NWURke5O6STkhyVHepD5krg@mail.gmail.com> <CANCZdfpshqwM_XvE6BUDhi48gwe2475PZG_9ZxGT2yu_611=RQ@mail.gmail.com>
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On 5/15/22 17:31, Warner Losh wrote: > Scratch that, this is a clang 11 vs clang13 issue. > > Most likely clang 13 is more aggressively optimizing, so fstat is not > actually on the stack... Because > of LTO, you'll need to check the disassembled binary to know for sure. > It makes no difference which compiler I use, same results either way. The disassembler is slightly different clang 11 / FreeBSD 13 201ba1: 8b bd b4 f6 ff ff mov -0x94c(%rbp),%edi 201ba7: 48 8d b5 d0 f6 ff ff lea -0x930(%rbp),%rsi 201bae: 89 85 a4 f6 ff ff mov %eax,-0x95c(%rbp) 201bb4: e8 f7 00 00 00 call 201cb0 <fstatfs@plt> clang 13 / FreeBSD 13.1 201b8d: 8b bd b4 f6 ff ff mov -0x94c(%rbp),%edi 201b93: 48 8d b5 d0 f6 ff ff lea -0x930(%rbp),%rsi clang 13 hasn't bothered emitting the mov %eax, which is passing an uninitialized local to deliberately generate a memcheck error. Looking at libc, I see for FreeBSD 13 1372c4: e8 87 89 f5 ff call 8fc50 <__getosreldate> 1372c9: 3d 9f 4f 12 00 cmp $0x124f9f,%eax 1372ce: 7c 10 jl 1372e0 <_fstatfs+0x40> HERE1 1372d0: 44 89 f7 mov %r14d,%edi 1372d3: 48 89 de mov %rbx,%rsi 1372d6: e8 85 d1 09 00 call 1d4460 <__sys_fstatfs@plt> 1372db: e9 22 01 00 00 jmp 137402 <_fstatfs+0x162> TO EXIT SEQUENCE HERE1: 1372e0: 48 8d 95 08 fe ff ff lea -0x1f8(%rbp),%rdx 1372e7: bf 8d 01 00 00 mov $0x18d,%edi 1372ec: 44 89 f6 mov %r14d,%esi 1372ef: 31 c0 xor %eax,%eax 1372f1: e8 8a c5 09 00 call 1d3880 <syscall@plt> 1372f6: 85 c0 test %eax,%eax 1372f8: 0f 85 04 01 00 00 jne 137402 <_fstatfs+0x162> and 13.1 2b4: e8 a7 c1 f5 ff call 90460 <__getosreldate> 1342b9: 3d 9f 4f 12 00 cmp $0x124f9f,%eax 1342be: 7c 25 jl 1342e5 <_fstatfs+0x55> HERE1 1342c0: 49 8b 07 mov (%r15),%rax 1342c3: 48 3b 45 e0 cmp -0x20(%rbp),%rax 1342c7: 0f 85 51 01 00 00 jne 13441e <_fstatfs+0x18e> 1342cd: 44 89 f7 mov %r14d,%edi 1342d0: 48 89 de mov %rbx,%rsi 1342d3: 48 81 c4 e8 01 00 00 add $0x1e8,%rsp 1342da: 5b pop %rbx 1342db: 41 5e pop %r14 1342dd: 41 5f pop %r15 1342df: 5d pop %rbp BINGO! TAIL CALL OPTIMIZATION 1342e0: e9 bb a0 09 00 jmp 1ce3a0 <__sys_fstatfs@plt> HERE1 1342e5: 48 8d 95 08 fe ff ff lea -0x1f8(%rbp),%rdx 1342ec: bf 8d 01 00 00 mov $0x18d,%edi 1342f1: 44 89 f6 mov %r14d,%esi 1342f4: 31 c0 xor %eax,%eax 1342f6: e8 d5 94 09 00 call 1cd7d0 <syscall@plt> 1342fb: 85 c0 test %eax,%eax 1342fd: 0f 85 04 01 00 00 jne 134407 <_fstatfs+0x177> I don't think that there is much that I can do about that in Valgrind. The stat family callstacks will be a little less clear, but still usable. And equally I don't suppose there's any chance that libc will get changed just for Valgrind. A+ Paul
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