First, I want to thank Super Bisquit, Fernando and Chris for their inputs. Second, the ps_strings struct remains in use to report information about the running process back to the user and operating system, and as such enriches the content of the FreeBSD kernel, so it's worth create this man page. However, I'll consult this with a member of the doc team so that everything is in order.
Attentively --CJPM 2013/8/19 Chris Torek <to...@torek.net> > >Despite I made a request not long ago[1], I'm looking for > >documentation to create the ps_strings structure man page because > >isn't covered in other man page such e.g. execve(2). So, I'm > >interested to know for what it's currently used. > > Nothing. (Well, backwards compatibility, depending on how far > backwards you go.) > > I invented the "ps_strings" struct a long time ago, about the same > time libkvm was new. > > Some background: There was code in "ps" that would grub around in > the top stack page of each user process and extract the argv > strings. This code knew how execve() worked inside the kernel > (copying the argv and environment strings into the user stack, > just below the signal trampoline code, and then setting up argv > and envp pointers and invoking the libc/csu "start program" code > at the entry point). > > We moved this grub-around-in-process-stack code to libkvm, but it > was still rather horrible code. > > Meanwhile, we had programs like sendmail that would "set their > process title" by saving, in some secret global variable, the > space where the "argv" array itself and its strings lived, and > then -- in setproctitle() -- carefully overwrite it. Of course > there was only as much room there as the kernel provided, based on > the actual strings at execve() time. > > I figured this would all be much cleaner if we created a small > data structure, namely "ps_strings", to hold the information that > libkvm would extract (and hence, ps would show). It would be > simpler than the original code, because the ps_strings structure > lived at a fixed, well known user-space virtual address (the same > VA in every process). Moreover, a user program (like sendmail) > could modify the ps_strings data to point to any other user-space > area: libkvm was smart enough to extract arbitrary data (and > verify the validity of the address too). This removed the limit > on how large a "process title" could be. > > FreeBSD now, however, uses a per-process p_args field in the > "proc" structure, with sysctl()s to set and get p_args. (I had > nothing to do with this new code, but I approve, as if anyone > cares. :-) ) This removes the fixed-virtual-address limitation. > The cost is a bit more kernel code (for the sysctl()s) and this > per-process data, but there is no more messing-about with "where > is ps_strings in this memory-layout / emulation" etc. (Meanwhile > libkvm still retrieves the arguments. It just does it now with > sysctl().) > > Chris > _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
