Re: [PATCH for-8.1] linux-user: Define real MAP_FIXED_NOREPLACE value

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2023/08/08 23:42, Richard Henderson wrote:
> On 8/8/23 04:52, Akihiko Odaki wrote:
>> do_brk() assumes target_mmap() emulates MAP_FIXED_NOREPLACE even when
>> the host does not support it. However, such emulation is not possible
>> if MAP_FIXED_NOREPLACE is defined as zero.
>>
>> Define MAP_FIXED_NOREPLACE with the real value instead of zero if it is
>> not defined.
>>
>> Fixes: e69e032d1a ("linux-user: Use MAP_FIXED_NOREPLACE for do_brk()")
>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
>> ---
>>   include/qemu/osdep.h | 8 ++++++--
>>   linux-user/elfload.c | 1 -
>>   2 files changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
>> index cc61b00ba9..1aac17ec2f 100644
>> --- a/include/qemu/osdep.h
>> +++ b/include/qemu/osdep.h
>> @@ -289,8 +289,12 @@ void QEMU_ERROR("code path is reachable")
>>   #ifndef MAP_ANONYMOUS
>>   #define MAP_ANONYMOUS MAP_ANON
>>   #endif
>> -#ifndef MAP_FIXED_NOREPLACE
>> -#define MAP_FIXED_NOREPLACE 0
>> +#if defined(__linux__) && !defined(MAP_FIXED_NOREPLACE)
>> +#if HOST_ALPHA
> 
> HOST_ALPHA is not a thing.  Also, see
> 
> https://lore.kernel.org/qemu-devel/20230808120303.585509-3-pbonzini@redhat.com/
> 
> where host support for Alpha is completely removed.

I sent v2 with the condition HOST_ALPHA replaced with
MAP_HUGETLB == 0x100000.

While Alpha is no longer supported, and linux-user will not work on 
Alpha, code outside linux-user also refers to osdep.h and the build 
script does not actively block building QEMU on an unsupported host so I 
left the definition for Alpha just in case.

> 
> 
>> +#define MAP_FIXED_NOREPLACE 0x200000
>> +#else
>> +#define MAP_FIXED_NOREPLACE 0x100000
>> +#endif
> 
> Which supported hosts do not define this value?  Can we simply remove 
> the fallback?

glibc didn't have this defined until 2.28. The older releases still 
maintained are 2.26, and 2.27, according to:
https://sourceware.org/glibc/wiki/Release

The page says ALT Linux p9 and Ubuntu 18.04 LTS (Bionic Beaver) has 
glibc 2.27.

> 
> 
> r~