3 ms·

How many warnings do you want for this small function? void oh_the_humanity(int ptr, int val) { ptr = val + 1; } Off the top of my head: * UB: ptr

How many warnings do you want for this small function? void oh_the_humanity(int *ptr, int val) { *ptr = val + 1; } Off the top of my head: * UB: ptr may be pointing a float variable. (It's not illegal to assign a float* to an int*, it's only UB when you actually dereference it with the wrong type.) * UB: val + 1 may overflow. * UB: potential data race on writing *ptr. * UB: ptr may be a one-past-the-end-of-the-array pointer, which can be validly constructed, but may not be dereferenced. * UB: ptr may be pointing to an object whose lifetime has expired. * UB: ptr may be uninitialized. * UB: val may be uninitialized. As you can see, UB is intensely specific to the actual data values; it's not really possible to catch even a large fraction of UB statically without severe false positive rates.

mzs 5y ago
Yeah I know I get it, it's me more being wishful, but I more seriously wish at least compilers could emit a warning when they optimize something after UB: % cat foo.c #include <stdlib.h> int foo(int *bar) { int baz = *bar; if (bar == NULL) exit(2); return (baz); } % cc -O3 -Wall -Wextra -c foo.c % objdump -dr foo.o foo.o: file format Mach-O 64-bit x86-64 Disassembly of section __TEXT,__text: 0000000000000000 _foo: 0: 55 pushq %rbp 1: 48 89 e5 movq %rsp, %rbp 4: 8b 07 movl (%rdi), %eax 6: 5d popq %rbp 7: c3 retq % cc -O0 -Wall -Wextra -c foo.c % objdump -dr foo.o foo.o: file format Mach-O 64-bit x86-64 Disassembly of section __TEXT,__text: 0000000000000000 _foo: 0: 55 pushq %rbp 1: 48 89 e5 movq %rsp, %rbp 4: 48 83 ec 10 subq $16, %rsp 8: 48 89 7d f8 movq %rdi, -8(%rbp) c: 48 8b 45 f8 movq -8(%rbp), %rax 10: 8b 08 movl (%rax), %ecx 12: 89 4d f4 movl %ecx, -12(%rbp) 15: 48 83 7d f8 00 cmpq $0, -8(%rbp) 1a: 0f 85 0a 00 00 00 jne 10 <_foo+0x2a> 20: bf 02 00 00 00 movl $2, %edi 25: e8 00 00 00 00 callq 0 <_foo+0x2a> 0000000000000026: X86_64_RELOC_BRANCH _exit 2a: 8b 45 f4 movl -12(%rbp), %eax 2d: 48 83 c4 10 addq $16, %rsp 31: 5d popq %rbp 32: c3 retq % That's very similar to something that bit me in embedded except it was with pointer to structure. Compiler realizes I've derefed NULL and that's UB anyway so no need to do the NULL check later and merrily scribble exc vectors or whatever.
- foxfluff 5y ago
  That's a nice example. It'd definitely be nice to have a warning for this one. Fwiw GCC does have a related warning flag (-Wnull-dereference) but I'm not sure it's made exactly for this. I believe it works based on functions being annotated for possibly returning NULL, e.g. malloc. It's also not enabled by -Wall or -W because apparently there were too many false positives: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96554 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96554 I imagine patches would be welcome. I'm guessing there are more people who like to wish for more compiler features than there are people who like to develop compilers :)
  - mzs 5y ago
    edit: Thanks, I just checked and the warning doesn't work https://godbolt.org/z/4TP1hfx4j https://godbolt.org/z/4TP1hfx4j But your hint found -fno-delete-null-pointer-checks which does the trick https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html And -fno-delete-null-pointer-checks made it into LLVM too. It's a good to know but a little late for when I needed it, cheers :) % cc -O3 -fno-delete-null-pointer-checks -Wall -Wextra -c foo.c % objdump -dr foo.o foo.o: file format Mach-O 64-bit x86-64 Disassembly of section __TEXT,__text: 0000000000000000 _foo: 0: 55 pushq %rbp 1: 48 89 e5 movq %rsp, %rbp 4: 48 85 ff testq %rdi, %rdi 7: 74 04 je 4 <_foo+0xd> 9: 8b 07 movl (%rdi), %eax b: 5d popq %rbp c: c3 retq d: bf 02 00 00 00 movl $2, %edi 12: e8 00 00 00 00 callq 0 <_foo+0x17> 0000000000000013: X86_64_RELOC_BRANCH _exit
    - astrange 5y ago
      If you want a general approach you can turn on ubsan in trap-only mode and see what traps have ended up in your output.
- zajio1am 5y ago
Dylan16807 5y ago
Well if we're not super worried about implementation difficulty right now: * 1 and 4-7: Don't worry about where the pointer goes, as long as you treat it like an int in this function. If there are going to be warnings about improper pointers, they should be at the call sites. * 2 If overflow will either trap, wrap, or behave like a bignum, then it's not the dangerous kind of UB, so no warning by default. Consider extending C so the coder can more easily control integer overflow. * 3 Anything could race. Out of scope, don't worry about it.

How many warnings do you want for this small function? void oh_the_humanity(int *ptr, int val) { *ptr = val + 1; } Off the top of my head: * UB: ptr

mzs 5y ago

Yeah I know I get it, it's me more being wishful, but I more seriously wish at least compilers could emit a warning when they optimize something after UB: % cat foo.c #include <stdlib.h> int foo(int *bar) { int baz = *bar; if (bar == NULL) exit(2); return (baz); } % cc -O3 -Wall -Wextra -c foo.c % objdump -dr foo.o foo.o: file format Mach-O 64-bit x86-64 Disassembly of section __TEXT,__text: 0000000000000000 _foo: 0: 55 pushq %rbp 1: 48 89 e5 movq %rsp, %rbp 4: 8b 07 movl (%rdi), %eax 6: 5d popq %rbp 7: c3 retq % cc -O0 -Wall -Wextra -c foo.c % objdump -dr foo.o foo.o: file format Mach-O 64-bit x86-64 Disassembly of section __TEXT,__text: 0000000000000000 _foo: 0: 55 pushq %rbp 1: 48 89 e5 movq %rsp, %rbp 4: 48 83 ec 10 subq $16, %rsp 8: 48 89 7d f8 movq %rdi, -8(%rbp) c: 48 8b 45 f8 movq -8(%rbp), %rax 10: 8b 08 movl (%rax), %ecx 12: 89 4d f4 movl %ecx, -12(%rbp) 15: 48 83 7d f8 00 cmpq $0, -8(%rbp) 1a: 0f 85 0a 00 00 00 jne 10 <_foo+0x2a> 20: bf 02 00 00 00 movl $2, %edi 25: e8 00 00 00 00 callq 0 <_foo+0x2a> 0000000000000026: X86_64_RELOC_BRANCH _exit 2a: 8b 45 f4 movl -12(%rbp), %eax 2d: 48 83 c4 10 addq $16, %rsp 31: 5d popq %rbp 32: c3 retq % That's very similar to something that bit me in embedded except it was with pointer to structure. Compiler realizes I've derefed NULL and that's UB anyway so no need to do the NULL check later and merrily scribble exc vectors or whatever.

foxfluff 5y ago
That's a nice example. It'd definitely be nice to have a warning for this one. Fwiw GCC does have a related warning flag (-Wnull-dereference) but I'm not sure it's made exactly for this. I believe it works based on functions being annotated for possibly returning NULL, e.g. malloc. It's also not enabled by -Wall or -W because apparently there were too many false positives: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96554 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96554 I imagine patches would be welcome. I'm guessing there are more people who like to wish for more compiler features than there are people who like to develop compilers :)
- mzs 5y ago
  edit: Thanks, I just checked and the warning doesn't work https://godbolt.org/z/4TP1hfx4j https://godbolt.org/z/4TP1hfx4j But your hint found -fno-delete-null-pointer-checks which does the trick https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html And -fno-delete-null-pointer-checks made it into LLVM too. It's a good to know but a little late for when I needed it, cheers :) % cc -O3 -fno-delete-null-pointer-checks -Wall -Wextra -c foo.c % objdump -dr foo.o foo.o: file format Mach-O 64-bit x86-64 Disassembly of section __TEXT,__text: 0000000000000000 _foo: 0: 55 pushq %rbp 1: 48 89 e5 movq %rsp, %rbp 4: 48 85 ff testq %rdi, %rdi 7: 74 04 je 4 <_foo+0xd> 9: 8b 07 movl (%rdi), %eax b: 5d popq %rbp c: c3 retq d: bf 02 00 00 00 movl $2, %edi 12: e8 00 00 00 00 callq 0 <_foo+0x17> 0000000000000013: X86_64_RELOC_BRANCH _exit
  - astrange 5y ago
    If you want a general approach you can turn on ubsan in trap-only mode and see what traps have ended up in your output.
zajio1am 5y ago

Dylan16807 5y ago

Well if we're not super worried about implementation difficulty right now: * 1 and 4-7: Don't worry about where the pointer goes, as long as you treat it like an int in this function. If there are going to be warnings about improper pointers, they should be at the call sites. * 2 If overflow will either trap, wrap, or behave like a bignum, then it's not the dangerous kind of UB, so no warning by default. Consider extending C so the coder can more easily control integer overflow. * 3 Anything could race. Out of scope, don't worry about it.