Re: [Regression] Bug 216911 - get_unmapped_area returns addresses below mmap_min_addr

["Linux kernel regression tracking (Thorsten Leemhuis)" <regressions@leemhuis.info>]

[resent with akpm's proper email address (either me or my mailer did
something stupid; sorry for the noise)]

On 11.01.23 11:46, Linux kernel regression tracking (Thorsten Leemhuis)
wrote:
> Hi, I noticed a regression report in bugzilla.kernel.org. As many
> (most?) kernel developer don't keep an eye on it, I decided to forward
> it by mail. Quoting from
> https://bugzilla.kernel.org/show_bug.cgi?id=216911 :
> 
>>  Amanieu d'Antras 2023-01-11 02:10:02 UTC
>>
>> Created attachment 303572 [details]
>> Program which demonstrates this issue
>>
>> The vm_unmapped_area function doesn't seem to respect info.low_limit and will sometimes return an address below this limit. This can result in addresses below mmap_min_addr being returned.
>>
>> I bisected it down to this range of commits (I stopped since I was hitting kernel crashes):
>> 3499a13168da mm/mmap: use maple tree for unmapped_area{_topdown}
>> 7fdbd37da5c6 mm/mmap: use the maple tree for find_vma_prev() instead of the rbtree
>> be8432e7166e mm/mmap: use the maple tree in find_vma() instead of the rbtree.
>> 2e3af1db1744 mmap: use the VMA iterator in count_vma_pages_range()
>> f39af05949a4 mm: add VMA iterator
>> d4af56c5c7c6 mm: start tracking VMAs with maple tree
> 
> [those are all from the Maple Tree introduction series]
> 
>> I've attached a test program which attempts to allocate the entire address space. On affected kernels, this triggers one of the 2 assertions (depending on whether you are running as root):
>> - NULL is returned, which means memory below mmap_min_addr was allocated.
>> - EPERM is returned, which really shouldn't happen when MAP_FIXED is not used (ENOMEM should be returned instead).
> 
> See the ticket for more details.
> 
> BTW, let me use this mail to also add the report to the list of tracked
> regressions to ensure it's doesn't fall through the cracks:
> 
> #regzbot introduced: d4af56c5c7c6..3499a13168da
> https://bugzilla.kernel.org/show_bug.cgi?id=216911
> #regzbot title: mm: get_unmapped_area returns addresses below mmap_min_addr
> #regzbot ignore-activity
> 
> Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
> --
> Everything you wanna know about Linux kernel regression tracking:
> https://linux-regtracking.leemhuis.info/about/#tldr
> If I did something stupid, please tell me, as explained on that page.