| Message ID | YOBIRA8oXQh4Antq@localhost.localdomain (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | cc-can-link.sh: check for linking capability in a more robust way | expand |
On Sat, Jul 3, 2021 at 8:21 PM Alexey Dobriyan <adobriyan@gmail.com> wrote: > > Compiling "printf("");" doesn't necessarily check for linking capability > as printf can be optimised for constants strings even at -O0: > > 0000000000401106 <main>: > 401106: push rbp > 401107: mov rbp,rsp > 40110a: mov eax,0x0 > 40110f: pop rbp > 401110: ret > > Pass something from the command line to disable optimisations: > > 0000000000401126 <main>: > 401126: push rbp > 401127: mov rbp,rsp > 40112a: sub rsp,0x10 > 40112e: mov DWORD PTR [rbp-0x4],edi > 401131: mov QWORD PTR [rbp-0x10],rsi > 401135: mov rax,QWORD PTR [rbp-0x10] > 401139: add rax,0x8 > 40113d: mov rax,QWORD PTR [rax] > 401140: mov rdi,rax > 401143: mov eax,0x0 > 401148: *** call 401030 <printf@plt> > 40114d: mov eax,0x0 > 401152: leave > 401153: ret > > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com> > --- > > scripts/cc-can-link.sh | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > --- a/scripts/cc-can-link.sh > +++ b/scripts/cc-can-link.sh > @@ -3,9 +3,9 @@ > > cat << "END" | $@ -x c - -o /dev/null >/dev/null 2>&1 > #include <stdio.h> > -int main(void) > +int main(int argc, char *argv[]) > { > - printf(""); > + printf(argv[1]); > return 0; > } > END Ah, right. But, we should not merge a bad coding example. argv[1] may contain '%' format string, and recent GCC versions warn about it. $ cat test.c #include <stdio.h> int main(int argc, char *argv[]) { printf(argv[1]); return 0; } $ gcc -c -o test.o test.c test.c: In function ‘main’: test.c:5:5: warning: format not a string literal and no format arguments [-Wformat-security] 5 | printf(argv[1]); | ^~~~~~ I think replacing printf("") with printf("a") (or any string you like) is enough.
On Tue, Jul 06, 2021 at 01:33:45AM +0900, Masahiro Yamada wrote: > On Sat, Jul 3, 2021 at 8:21 PM Alexey Dobriyan <adobriyan@gmail.com> wrote: > > > > Compiling "printf("");" doesn't necessarily check for linking capability > > as printf can be optimised for constants strings even at -O0: > > > > 0000000000401106 <main>: > > 401106: push rbp > > 401107: mov rbp,rsp > > 40110a: mov eax,0x0 > > 40110f: pop rbp > > 401110: ret > > > > Pass something from the command line to disable optimisations: > > > > 0000000000401126 <main>: > > 401126: push rbp > > 401127: mov rbp,rsp > > 40112a: sub rsp,0x10 > > 40112e: mov DWORD PTR [rbp-0x4],edi > > 401131: mov QWORD PTR [rbp-0x10],rsi > > 401135: mov rax,QWORD PTR [rbp-0x10] > > 401139: add rax,0x8 > > 40113d: mov rax,QWORD PTR [rax] > > 401140: mov rdi,rax > > 401143: mov eax,0x0 > > 401148: *** call 401030 <printf@plt> > > 40114d: mov eax,0x0 > > 401152: leave > > 401153: ret > > > > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com> > > --- > > > > scripts/cc-can-link.sh | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > --- a/scripts/cc-can-link.sh > > +++ b/scripts/cc-can-link.sh > > @@ -3,9 +3,9 @@ > > > > cat << "END" | $@ -x c - -o /dev/null >/dev/null 2>&1 > > #include <stdio.h> > > -int main(void) > > +int main(int argc, char *argv[]) > > { > > - printf(""); > > + printf(argv[1]); > > return 0; > > } > > END > > Ah, right. > > But, we should not merge a bad coding example. > > argv[1] may contain '%' format string, and > recent GCC versions warn about it. > > > > $ cat test.c > #include <stdio.h> > > int main(int argc, char *argv[]) > { > printf(argv[1]); > return 0; > } > $ gcc -c -o test.o test.c > test.c: In function ‘main’: > test.c:5:5: warning: format not a string literal and no format > arguments [-Wformat-security] > 5 | printf(argv[1]); > | ^~~~~~ > I think replacing printf("") with printf("a") > (or any string you like) > is enough. I get putchar() for "a". puts(argv[1]) works too. I think argv[1] should be used to defeat optimisers, current and future.
--- a/scripts/cc-can-link.sh +++ b/scripts/cc-can-link.sh @@ -3,9 +3,9 @@ cat << "END" | $@ -x c - -o /dev/null >/dev/null 2>&1 #include <stdio.h> -int main(void) +int main(int argc, char *argv[]) { - printf(""); + printf(argv[1]); return 0; } END
Compiling "printf("");" doesn't necessarily check for linking capability as printf can be optimised for constants strings even at -O0: 0000000000401106 <main>: 401106: push rbp 401107: mov rbp,rsp 40110a: mov eax,0x0 40110f: pop rbp 401110: ret Pass something from the command line to disable optimisations: 0000000000401126 <main>: 401126: push rbp 401127: mov rbp,rsp 40112a: sub rsp,0x10 40112e: mov DWORD PTR [rbp-0x4],edi 401131: mov QWORD PTR [rbp-0x10],rsi 401135: mov rax,QWORD PTR [rbp-0x10] 401139: add rax,0x8 40113d: mov rax,QWORD PTR [rax] 401140: mov rdi,rax 401143: mov eax,0x0 401148: *** call 401030 <printf@plt> 40114d: mov eax,0x0 401152: leave 401153: ret Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com> --- scripts/cc-can-link.sh | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)