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[[!tag open_issue_binutils]]

Have a look at GOLD / port as needed.


# teythoon's try / `mremap` issue

IRC, #hurd, 2011-01-12

    <teythoon> I've been looking into building gold on hurd and it built fine
      with one minor tweak
    <teythoon> and it's working fine according to its test suite
    <teythoon> the only problem is that the build system is failing to detect
      the hurdish mremap which lives in libmemusage
    <teythoon> on linux it is in the libc so the check succeeds
    <teythoon> any hints on how to fix this properly?
    <antrik> hm... it's strange that it's a different library on the Hurd
    <antrik> are the implementations compatible?
    <teythoon> antrik: it seems so, though the declarations differ slightly
    <antrik> I guess the best thing is to ask on the appropriate list(s) why
      they are different...
    <teythoon> teythoon@ganymede:~/build/gold/binutils-2.21/gold$ grep -A1
      mremap /usr/include/sys/mman.h 
    <teythoon> extern void *mremap (void *__addr, size_t __old_len, size_t
      __new_len, int __flags, ...) __THROW;
    <teythoon> vs
    <antrik> of course it would be possible to modify the configure script to
      check for the Hurd variant too; but first we should establish whether
      here is actually any reason for being different, or it's just some
      historical artefact that should be fixed...
    <teythoon> teythoon@ganymede:~/build/gold/binutils-2.21/gold$ fgrep 'extern
      void *mremap' mremap.c 
    <teythoon> extern void *mremap (void *, size_t, size_t, int, ...);
    <teythoon> the problem is that the test fails to link due to the fact that
      mremap isn't in the libc on hurd
    <antrik> yeah, it would be possible for the configure script to check
      whether it works when the hurdish extra library is added explicitely
    <antrik> but again, I don't see any good reason for being different here in
      the first place...
    <teythoon> so should I create a patch to move mremap?
    <antrik> if it's not too complicated, that would be nice... it's always
      easier to discuss when you already have code :-)
    <antrik> OTOH, asking first might spare you some useless work if it turns
      out there *is* some reason for being different after all...
    so where is the right place to discuss this?
    <antrik> bug-hurd mailing list and/or glibc mailing list. not sure which
      one is better -- I guess it doesn't hurt to crosspost...

[[mailing_lists/libc-alpha]] is the correct list, and cross-posting to
[[mailing_lists/bug-hurd]] would be fine, too.

    <teythoon> antrik: some further digging revealed that mremap belongs to
      /lib/libmemusage.so on both hurd and linux
    <teythoon> the only difference is that on linux there is a weak reference
      to that function in /lib/libc-2.11.2.so
    <teythoon> $ objdump -T /lib/libc-2.11.2.so | fgrep mremap
    <teythoon> 00000000000cf7e0  w   DF .text  0000000000000028  GLIBC_2.2.5
      mremap
    <antrik> ah, it's probably simply a bug that we don't have this weak
      reference too
    <antrik> IIRC we had similar bugs before
    <antrik> teythoon: can you provide a patch for that?
    <teythoon> antrik: unfortunately I have no idea how that weak ref ended up
      there

    <guillem> teythoon: also the libmemusage.s seems to be just a debugging
      library to be used by LD_PRELOAD or similar
    <guillem> which override those memory functions
    <guillem> the libc should provide actual code for those functions, even if
      the symbol is declared weak (so overridable)
    <guillem> teythoon: are you sure that's the actual problem? can you paste
      somewhere the build logs with the error?
    <teythoon> guillem: sure
    <teythoon> http://paste.debian.net/104437/
    <teythoon> that's the part of config.log that shows the detection (or the
      failure to detect it) of mremap
    <teythoon> this results in HAVE_MREMAP not being defined
    <teythoon> as a consequence it is declared in gold.h and this declaration
      conflicts with the one from sys/mman.h http://paste.debian.net/104438/
    <teythoon> on linux the test for mremap succeeds
    <guillem> teythoon: hmm oh I guess it's just what that, mremap is linux
      specific so it's not available on the hurd
    <guillem> teythoon: I just checked glibc and seems to confirm that
    <braunr> CONFORMING TO This call is Linux-specific, and should not be used
      in programs intended to be portable.
    <teythoon> ah okay
    <teythoon> so I guess we shouldn't ship an header with that declaration...
    <guillem> teythoon: yeah :/ good luck telling that to drepper :)
    <guillem> teythoon: I guess he'll suggest that everyone else needs to get
      our own copy of sys/mman.h
    <guillem> s/our/their/
    <teythoon> hm, so how should I proceed?
    <braunr> what's your goal ?
    <braunr> detecting mremap ?
    <teythoon> making binutils/gold compile ootb on hurd
    <teythoon> I picked it from the open issues page ;)
    <braunr> well, if there is no mremap, you need a replacement
    <teythoon> gold has a replacement
    <braunr> ok
    <braunr> so your problem is fixing the detection of mremap right ?
    <teythoon> yes
    <braunr> ok, that's a build system question then :/
    <braunr> you need to ask an autotools guy
    <teythoon> well, actually the build system correctly detects the absence of
      mremap
    <braunr> (gold does use the autotools right ?)
    <teythoon> yes
    <braunr> oh, i'm lost now (i admit i didn't read the whole issue :/)
    <teythoon> it is just that the declaration in sys/mman.h conflicts with
      their own declaration
    <braunr> ah
    <braunr> so in the absence of mremap, they use their own builtin function
    <teythoon> yes
    <teythoon> and according to the test suite it is working perfectly
    <teythoon> gold that is
    <teythoon> the declaration in mman.h has an extra __THROW
    <guillem> a workaround would be to rename gold's mremap to something else,
      gold_mremap for example
    <braunr> that's really the kind of annoying issue
    <braunr> you either have to change glibc, or gold
    <guillem> yeah
    <braunr> you'll face difficulty changing glibc, as guillem told you
    <guillem> the correct solution though IMO is to fix glibc
    <braunr> but this may be true for gold too
    <braunr> guillem: i agree
    <antrik> maybe it would be easiest actually to implement mremap()?...
    <braunr> but as this is something quite linux specific, it makes sense to
      use another internal name, and wrap that to the linux mremap if it's
      detected
    <braunr> antrik: i'm nto sure
    <antrik> braunr: I don't think using such workarounds is a good
      idea. clearly there would be no issue if the header file wouldn't be
      incorrect on Hurd
    <braunr> antrik: that's why i said i agree with guillem when he says "the
      correct solution though IMO is to fix glibc"
    <teythoon> what exactly is the problem with getting a patch into glibc?
    <braunr> the people involved
    <guillem> teythoon: and touching a generic header file
    <braunr> but feel free to try, you could be lucky
    <teythoon> but glibc is not an linux specific piece of software, right?
    <braunr> teythoon: no, it's not
    <guillem> erm...
    <braunr> teythoon: but in practice, it is
    <guillem> supposedly not :)
    <antrik> braunr: BTW, by "easiest" I don't mean coding alone, but
      coding+pushing upstream :-)
    <guillem> so the problem is, misc/sys/mman.h should be a generic header and
      as such not include linux specific parts, which are not present on hurd,
      kfreebsd, etc etc
    <braunr> antrik: yes, that's why guillem and i suggested the workaround
      thing in gold
    <antrik> that also requires pushing upstream. and quite frankly, if I were
      the gold maintainer, I wouldn't accept it.
    <guillem> but the easiest (and wrong) solution in glibc to avoid maintainer
      conflict will probably be copying that file under hurd's glibc tree and
      install that instead
    <braunr> antrik: implementing mremap could be relatively easy to do
      actually
    <braunr> antrik: IIRC, vm_map() supports overlapping
    <antrik> well, actually the easiest solution would be to create a patch
      that never goes upstream but is included in Debian, like many
      others... but that's obviously not a good long-term plan
    <antrik> braunr: yes, I think so too
    <antrik> braunr: haven't checked, but I have a vague recollection that the
      fundamentals are pretty much there
    <antrik> teythoon: so, apart from an ugly workaround in gold, there are
      essentially three options: 1. implement mremap; 2. make parts of mman.h
      conditional; 3. use our own copy of mman.h
    <antrik> 1. would be ideal, but might be non-trivial; 2. would might be
      tricky to get right, and even more tricky to get upstream; 3. would be
      simple, but a maintenance burden in the long term
    <teythoon> looking at golds replacement code (mmap & memcpy) 1 sounds like
      the best option performance wise

[[!taglink open_issue_glibc]]: check if it is possible to implement `mremap`.
[[I|tschwinge]] remember some discussion about this, but have not yet worked on
locating it.  [[Talk to me|tschwinge]] if you'd like to have a look at this.