Re: Marking a Page of Memory Executable

On Jun 4, 7:29 pm, jacob navia <ja...@xxxxxxxxxx> wrote:
Pie Squared wrote:
I'm not completely sure that this is the right place to ask, but I'm
doing it in C, so I'm asking, but if I'm wrong, then please don't
hesitate to correct me and tell me where to post this.

What I want to do is get an executable and writable page of memory, so
that I can (say) write machine code to it and then switch %eip (the
instruction pointer on x86) to that page so that it will execute that
code, or something similar.

I'm attempting to figure out how JIT's manage to run code made during
run-time without writing to an executable file, so if anyone knows
that or has some suggestions I'd like to hear those too.

I'm using Ubuntu Hardy Heron, by the way, in case that matters. I
tried looking in comp.os.linux, but that seems to be archived, so I
can't post this there.

Thanks, and sorry if this is mis-posted.

--Piesquared

Hi Piesquared (pipi for friends I suppose :-)

% man mmap

MMAP(2)             Linux Programmer's Manual             MMAP(2)

NAME
        mmap, munmap - map or unmap files or devices into memory

SYNOPSIS
        #include <unistd.h>
        #include <sys/mman.h>

        #ifdef _POSIX_MAPPED_FILES

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

        int munmap(void *start, size_t length);

        #endif

DESCRIPTION
        The mmap function asks to map  length  bytes  starting  at
        offset offset from the file (or other object) specified by
        the file descriptor fd into memory, preferably at  address
        start.  This latter address is a hint only, and is usually
        specified as 0.  The actual  place  where  the  object  is
        mapped is returned by mmap, and is never 0.

        The  prot argument describes the desired memory protection
        (and must not conflict with the open mode of the file). It
        is either PROT_NONE or is the bitwise OR of one or more of
        the other PROT_* flags.

        PROT_EXEC  Pages may be executed.

etc, did not copy the rest

--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatiquehttp://www.cs.virginia.edu/~lcc-win32

Thanks, all! That was immensely useful.

Thanks, Dave, for that info. I'll keep that in mind from now on
whenever I post here (I'm sort-of new to USENET). Thanks for helping a
newbie in need. :)

Also, so _that's_ what the infamous mmap does...

And now let this topic sink into obscurity before my mispost angers
anyone. ;-)
.

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