23 Oct
2017
23 Oct
'17
7:11 a.m.
On Sun, Oct 22, 2017 at 7:00 PM, Doug McIlroy <doug(a)cs.dartmouth.edu> wrote:
Oh sure! Sorry; the mention of the 4K data section was meant to be
orthogonal to the empty-shell file thing. However, one still cannot
'exec' an empty shell script, even on OS X:
: hurricane; mkdir t
: hurricane; cd t
/Users/dcross/t
: hurricane; touch true
: hurricane; cat>dotrue.c
int main() { execl("./true", "true", 0); write(2, "Boo\n",
4); return 1; }
: hurricane; chmod +x ./true
: hurricane; ./true
: hurricane; echo $?
0
: hurricane; make dotrue
cc dotrue.c -o dotrue
dotrue.c:1:14: warning: implicit declaration of function 'execl' is
invalid in C99 [-Wimplicit-function-declaration]
int main() { execl("./true", "true", 0); write(2, "Boo\n",
4); return 1; }
^
dotrue.c:1:42: warning: implicit declaration of function 'write' is
invalid in C99 [-Wimplicit-function-declaration]
int main() { execl("./true", "true", 0); write(2, "Boo\n",
4); return 1; }
^
2 warnings generated.
: hurricane; ./dotrue
Boo
: hurricane; echo $?
1
: hurricane; cp /usr/bin/true .
: hurricane; ./dotrue
: hurricane; echo $?
0
: hurricane; file true
true: Mach-O 64-bit executable x86_64
: hurricane; ls -l true
-rwxr-xr-x 1 dcross eng 17760 Oct 22 21:08 true
: hurricane; cd ..
/Users/dcross
: hurricane; rm -fr t
: hurricane;
(The system-provided /usr/bin/true is >16KB!!)
- Dan C.
macOS requires
you to have a data section aligned to 4K, even if you
don't use it. The resulting binary is a little over 8K; again, mostly
zeros.
Not quite. The classic empty executable file for /bin/true works
on OS X. That is not just a clever trick;it's a natural consequence
of Kernighan's ancient prrecept: do nothing gracefully. Conceivably
the 4K data section is, too--if the page has no physical presence
until it is accessed.