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

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

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:
>>
>> --- 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 :)

>> +                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;
>> +                }
>>               }
>>                 vaddr_ef = vaddr + eppnt->p_filesz;
>> --- END ---

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,
---

>>
>> -- System Information:
>> Debian Release: buster/sid
>>    APT prefers unstable-debug
>>    APT policy: (500, 'unstable-debug'), (500, 'stable-updates'), (500,
>> 'unstable'), (500, 'testing'), (500, 'stable'), (1, 'experimental')
>> Architecture: amd64 (x86_64)
>> Foreign Architectures: i386
>>
>> Kernel: Linux 4.19.0-1-amd64 (SMP w/4 CPU cores)
>> Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8),
>> LANGUAGE=en_GB.UTF-8 (charmap=UTF-8)
>> Shell: /bin/sh linked to /bin/dash
>> Init: systemd (via /run/systemd/system)
>> LSM: AppArmor: enabled
>>
>> Versions of packages qemu-user depends on:
>> ii  libc6         2.28-3
>> ii  libcapstone3  3.0.5-3
>> ii  libgcc1       1:8.2.0-13
>> ii  libglib2.0-0  2.58.1-2
>> ii  libstdc++6    8.2.0-13
>> ii  zlib1g        1:1.2.11.dfsg-1
>>
>> Versions of packages qemu-user recommends:
>> ii  qemu-user-static [qemu-user-binfmt]  1:3.1+dfsg-2
>>
>> Versions of packages qemu-user suggests:
>> ii  sudo  1.8.26-2
>>
>> -- no debconf information
>>
>