Re: [LTP] [PATCH] mmap/munmap /dev/zero

[Mike Frysinger <vapier@gentoo.org>]

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On Tuesday, October 12, 2010 22:06:29 CAI Qian wrote:
> ----- "Mike Frysinger" wrote:
> > On Tuesday, October 12, 2010 10:07:16 CAI Qian wrote:
> > > +	 x = mmap("/dev/zero", SIZE+SIZE-4096, PROT_READ|PROT_WRITE,
> > > +		 MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
> > 
> > uhh, have you even checked this test ?  this mmap() makes absolutely
> > no sense on so many levels.
> 
> Can you elaborate?

so you havent actually looked at the syscalls made by the application and the 
memory maps created nor used mmap() in detail before ?  well, let's start with 
the mmap() man page:

	void *mmap(void *addr, size_t length, int prot, int flags,
		int fd, off_t offset);

"addr" doesnt sound like "file name" to me, and the description agrees:

	If addr is not NULL, then the kernel takes it as a hint about where to
	place the mapping; on Linux, the mapping will be created at a nearby page
	boundary.  The address of the new mapping is returned as the result of
	the call.

so basically you told the kernel to create a mapping in/near your .rodata 
section (since that is the address of the constant string "/dev/zero").  the 
contents of that pointer (which is not of type "char *") have absolutely no 
meaning to the kernel.

then there are the flags you're using:

	MAP_ANONYMOUS
		The mapping is not backed by any file; its contents are initialized
		to zero.  The fd and offset arguments are ignored; however, some
		implementations require fd to be -1 if MAP_ANONYMOUS (or MAP_ANON)
		is specified, and portable applications should ensure this.

so not only are you not getting a mapping from /dev/zero, you're just 
allocating a random piece of memory that the kernel has guaranteed will be 
zeroed out for you.  so any attempts to verify the contents are zero *because 
the data came from /dev/zero* fail.

finally, there's the fd which you're passing as "-1":

	The contents of a file mapping (as opposed to an anonymous mapping; see
	MAP_ANONYMOUS below), are initialized using length bytes starting at
	offset "offset" in the file (or other object) referred to by the file
	descriptor "fd".

"-1" is not a valid fd, so there's no way it could possibly be from /dev/zero.

so we're back where we started: this test makes no sense as written and you 
apparently havent verified it "works" beyond "is the exit status 0 when it 
finished".

> > why are you using _exit() ?
> 
> It is used to terminate a child.

i meant why arent you using exit() ?  why do you need _exit() semantics ?
-mike

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