Re: [Qemu-devel] Bug#919921: qemu-user Linux ELF loader fails to handle pure BSS segments

[Ben Hutchings]

On Tue, 2019-01-22 at 21:31 -0800, Richard Henderson wrote:
> On 1/22/19 1:39 AM, Philippe Mathieu-Daudé wrote:
> > Hi Ben,
> > 
> > On 1/22/19 6:43 AM, Michael Tokarev wrote:
> > > Forwarding to qemu-devel@
> > > http://bugs.debian.org/919921
> > > 
> > > Thanks!
> > > 
> > > 20.01.2019 20:55, Ben Hutchings wrote:
> > > > Package: qemu-user
> > > > Version: 1:3.1+dfsg-2
> > > > Severity: normal
> > > > Tags: patch
> > > > 
> > > > I've been building and testing klibc across many architectures using
> > > > qemu-user, and I found that qemu-user fails to load a few programs on
> > > > a few architectures, reporting an EINVAL error code.  Here's the
> > > > "readelf -l" output for one such program:
> > > > 
> > > >      Elf file type is EXEC (Executable file)
> > > >      Entry point 0x10000100
> > > >      There are 5 program headers, starting at offset 52
> > > >           Program Headers:
> > > >        Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz 
> > > > Flg Align
> > > >        PHDR           0x000034 0x10000034 0x10000034 0x000a0 0x000a0
> > > > R   0x4
> > > >        INTERP         0x0000d4 0x100000d4 0x100000d4 0x0002a 0x0002a
> > > > R   0x1
> > > >            [Requesting program interpreter:
> > > > /lib/klibc-R7FVdnsTBUFpWPgCV6FR07b-mf8.so]
> > > >        LOAD           0x000000 0x10000000 0x10000000 0x002f8 0x002f8 R
> > > > E 0x10000
> > > >        LOAD           0x010000 0x10020000 0x10020000 0x00000 0x08000
> > > > RW  0x10000
> > > >        GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000
> > > > RWE 0x10
> > > >            Section to Segment mapping:
> > > >        Segment Sections...
> > > >         00
> > > >         01     .interp
> > > >         02     .interp .text .rodata .eh_frame
> > > >         03     .bss
> > > >         04
> > > > 
> > > > The unusual feature of this program, and all the others that failed,
> > > > is that there is a LOAD segment with a file-size of 0 (i.e.  only BSS,
> > > > no initialised data).  load_elf_image() will try to mmap() initialised
> > > > data for this section even though there is none and a length of 0 is
> > > > invalid.
> > > > 
> > > > The change that seems to fix this is to skip the mmap() in this case:

Is there a reason why my fix can't be applied?  Do I need to open a bug
report in Launchpad for this?

Ben.

> > > > --- a/linux-user/elfload.c
> > > > +++ b/linux-user/elfload.c
> > > > @@ -2316,11 +2316,13 @@ static void load_elf_image(const char *i
> > > >               vaddr_ps = TARGET_ELF_PAGESTART(vaddr);
> > > >               vaddr_len = TARGET_ELF_PAGELENGTH(eppnt->p_filesz +
> > > > vaddr_po);
> > > >   -            error = target_mmap(vaddr_ps, vaddr_len,
> > > > -                                elf_prot, MAP_PRIVATE | MAP_FIXED,
> > > > -                                image_fd, eppnt->p_offset - vaddr_po);
> > > > -            if (error == -1) {
> > > > -                goto exit_perror;
> > > > +            if (vaddr_len != 0) {
> > 
> > This is probably not the good fix, since now your process doesn't have
> > anything mapped to use his BSS :)
> 
> Not true.  The mapping happens in zero_bss.
> 
> > What about this fix instead, using the segment memory size rather than
> > the file size:
> > 
> > -- >8 --
> > @@ -2314,7 +2314,7 @@ static void load_elf_image(const char *image_name,
> > int image_fd,
> >              vaddr = load_bias + eppnt->p_vaddr;
> >              vaddr_po = TARGET_ELF_PAGEOFFSET(vaddr);
> >              vaddr_ps = TARGET_ELF_PAGESTART(vaddr);
> > -            vaddr_len = TARGET_ELF_PAGELENGTH(eppnt->p_filesz + vaddr_po);
> > +            vaddr_len = TARGET_ELF_PAGELENGTH(eppnt->p_memsz + vaddr_po);
> > 
> >              error = target_mmap(vaddr_ps, vaddr_len,
> >                                  elf_prot, MAP_PRIVATE | MAP_FIXED,
> 
> No, there's only filesz bytes in the file.  I'd expect zero_bss to map over 
> the
> extra that you just mapped, but it doesn't help.
> 
> 
> r~
-- 
Ben Hutchings - Debian developer, member of kernel, installer and LTS teams

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