Re: [Qemu-devel] [PATCH] linux-user: Do not subtract offset from end address

[Tom Musta <tommusta@gmail.com>]

On 11/8/2014 6:22 PM, Andreas Färber wrote:
> Am 06.11.2014 um 20:43 schrieb Tom Musta:
>> When computing the upper address of a program segment, do not subtract the
>> offset from the virtual address; instead compute the sum of the virtual address
>> and the memory size.
> 
> Note that this reads a bit weird as both old and new code are adding,
> not subtracting.
> 
> Regards,
> Andreas
> 

I agree that it is not obvious from the patch, which needed one more line of
context:

            abi_ulong a = phdr[i].p_vaddr - phdr[i].p_offset;
            if (a < loaddr) {
                loaddr = a;
            }
            a = phdr[i].p_vaddr + phdr[i].p_memsz;
            if (a > hiaddr) {
                hiaddr = a;
            }

I think the description accurately captures what is being changed in the code.
But if you still disagree, I will reword and respin V2.

>>
>> Signed-off-by: Tom Musta <tommusta@gmail.com>
>> ---
>>
>> Please include this patch in QEMU 2.2.  
>>
>> Commit a93934fecd4dffc9d4b452b670c9506be5dea30d injected a regression of Linux
>> User Mode that I was able to detect on PowerPC 64 (but not x86).  I suspect that
>> large page size on the host has something to do with it.  In any case, that commit
>> adjusted the lower address of a program segment by the program header's offset 
>> field.  However, it also inadvertantly adjusted the upper address by the offset also.
>>
>>  linux-user/elfload.c |    2 +-
>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/linux-user/elfload.c b/linux-user/elfload.c
>> index 84123ba..e2596a4 100644
>> --- a/linux-user/elfload.c
>> +++ b/linux-user/elfload.c
>> @@ -1824,7 +1824,7 @@ static void load_elf_image(const char *image_name, int image_fd,
>>              if (a < loaddr) {
>>                  loaddr = a;
>>              }
>> -            a += phdr[i].p_memsz;
>> +            a = phdr[i].p_vaddr + phdr[i].p_memsz;
>>              if (a > hiaddr) {
>>                  hiaddr = a;
>>              }
>>
> 
>