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[[!meta copyright="Copyright © 2011 Free Software Foundation, Inc."]]
[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
id="license" text="Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no Invariant
Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license
is included in the section entitled [[GNU Free Documentation
License|/fdl]]."]]"""]]
Several issues here:
* [[!tag open_issue_glibc open_issue_gnumach]] Even invalid `mmap` shoudn't
crash the process.
* [[!tag open_issue_documentation]] The memory layout example should be
documented.
* [[!tag open_issue_gnumach]] New `vm_map` allocation strategy may be
desirable; see also [[placement_of_virtual_memory_regions]].
* [[!tag open_issue_glibc]] *task X deallocating an invalid port Y, most
probably a bug*.
IRC, freenode, #hurd, 2011-08-11
< zyg> oh, mmap sigsegvs, strange.
< braunr> hwo do you see that ?
< zyg> braunr: I'll try to paste a minimal case
< braunr> zyg: make sure you have a sane memory setup
< braunr> 512 RAM / 1G swap seems good
< braunr> have more swap than RAM
< zyg> I have those. Still it shouldn't sigsegv.
< braunr> gnumach is picky about that
< braunr> and yes, the hurd shouldn't have bugs
< zyg> braunr: ready to crash? #include <stdio.h> #include <sys/mman.h> int
main (int argc, char **argv) { mmap(0x10000, 0x8000, PROT_READ, MAP_ANON
| MAP_FIXED, -1, 0); return 0; }
< braunr> a fixed mapping at such an address is likely to fail, yes
< braunr> but a crash, hm
< zyg> why should it fail?
< braunr> because the hurd doesn't have a common text data bss heap stack
layout
< braunr> e.g. there are mappings below text, as show by vminfo :
< braunr> $ vminfo $$
< braunr> 0[0x1000] (prot=0)
< braunr> 0x1000[0x21000] (prot=RX, max_prot=RWX, mem_obj=105)
< braunr> 0x22000[0x1000] (prot=R, max_prot=RWX, mem_obj=105)
< braunr> 0x23000[0x1000] (prot=RW, max_prot=RWX, mem_obj=105)
< braunr> 0x24000[0x1000] (prot=0, max_prot=RWX)
< braunr> 0x25000[0xfff000] (prot=RWX, mem_obj=106)
< braunr> 0x1024000[0x1000] (prot=RWX, mem_obj=107)
< braunr> 0x1025000[0x1000] (prot=RW, max_prot=RWX, mem_obj=108)
< braunr> 0x1026000[0x1000] (prot=RW, max_prot=RWX, mem_obj=108,
offs=0x1000)
< braunr> 0x1027000[0x1000] (prot=RW, max_prot=RWX, mem_obj=109)
< braunr> 0x1028000[0x2000] (prot=RW, max_prot=RWX, mem_obj=110,
offs=0x1000)
< braunr> 0x102a000[0x1000] (prot=RW, max_prot=RWX, mem_obj=111)
< braunr> (sorry for the long paste)
< zyg> oh.. my mmap falls into an occupied range?
< braunr> seems so
< zyg> thanks, that was really useful.
< braunr> MAP_FIXED isn't portable, this is clearly stated in most man
pages
< zyg> yes, implementation specific it says
< braunr> well the behaviour isn't specific, it's well defined, but the
memory layout isn't
< braunr> i personally think vm_map() should be slightly changed to include
a new flag for top-down allocations
< braunr> so that our stack and libraries are at high addresses, below the
kernel
< braunr> zyg: what kind of error do you get ? i don't get sigsegv
< zyg> I get both sigsegv and sigill depending on addr
< braunr> ok
< braunr> i get sigill with your example
< braunr> the error is the same (wrong memory access) but the behaviour
changes because of the special memory configuration
< zyg> yes.. I guess the usecase is too uncommon. Else mmap would have an
guard
< braunr> some accesses cause invalid page faults (which are sent as
segmentation faults) while other cause general protection faults (which
are sent as illegal instructions)
< braunr> (this is quite weird since the GP fault is likely because the
access targets something out of the data or code segment eh)
< zyg> braunr: that's very os-specific. Do you mean hurd behaves that way?
< braunr> gnumach
< braunr> on i386
< braunr> the segmant configuration isn't completely flat
< braunr> segment*
< braunr> hm nice
< braunr> your small program triggers the "task X deallocating an invalid
port Y, most probably a bug." message
< zyg> where do you see that?
< braunr> on the mach console
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