On Tue, 14 Nov 2017 14:18:42 +0300, Lev Olshvang said: > The difference between executable and file that executable may crash. while > shared lib can not. Oh, a shared lib can indeed crash (or more correctly cause a crash in the process that is using it). > Still there are unknown for me what happen with opened files and mmaped files > when crash occurs Same thing as an executable or a mapped shared library (.so's are just mmap() under the covers), Reference counts are reference counts. > I used to think that kernel decrease reference counts and closes files, > whether application exits normally or crashed. Right. And you change those reference counts on your own at your own peril. > Now I add some facts about executables from kernel code: > fss/binfmt_misc.c: deny_write_access(interp_file); > fs/exec.c: err = deny_write_access(file); > fs/exec.c: ret = deny_write_access(file); > > And I found following explanatioin in old kernel list archive: > https://lists.gt.net/linux/kernel/222875 > > The reason the kernel refuses to honour it, is that MAP_DENYWRITE is an > > > excellent DoS-vehicle - you just mmap("/etc/passwd") with MAP_DENYWRITE, > > > and even root cannot write to it.. Vary nasty. Right - so DENYWRITE is restricted to executables (where it makes sense anyhow) However, shared libraries are just mmap() - so there's no easy way to say "only allow DENYWRITE for .so images". (Hint - a shared library doesn't have to be called something.so - and in fact is usually 'something.so.versionstring") > And I still confused because shared libraries are mapped with PROT_EXEC flag > and so they differ > from regular file like /etc/passwd and generally have -r-x file system > permissions. Actually, most shared libraries will end up with several mmap() segments - one for .txt, one for .bss (uninitialized variables), and one for .data (initialized variables) - and they will be mapped with different flags. Do an 'strace /bin/echo foo' and ponder what actually happens.
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