linux-kernel - Re: MIPS: bug: gettimeofday syscall broken on CI20 board

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Date:   Thu, 28 Nov 2019 14:01:01 +0000
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     "H. Nikolaus Schaller" <hns@...delico.com>,
        Maarten ter Huurne <maarten@...ewalker.org>
Cc:     mips-creator-ci20-dev@...glegroups.com,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>, linux-mips@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>
Subject: Re: MIPS: bug: gettimeofday syscall broken on CI20 board

On 28/11/2019 13:48, H. Nikolaus Schaller wrote:
> 
>> Am 28.11.2019 um 14:29 schrieb Maarten ter Huurne <maarten@...ewalker.org>:
>>
>> On Thursday, 28 November 2019 13:33:17 CET H. Nikolaus Schaller wrote:
>>> Hi Vincenzo,
>>>
>>>> Am 28.11.2019 um 13:21 schrieb Vincenzo Frascino
>>>> <vincenzo.frascino@....com>:> 
>>>> [...]
>>>> The the lib that provides the gettimeofday() changes accordingly
>>>> with vdso_data. 5.4 and 4.19 have 2 different vdso libraries as
>>>> well.
>>>
>>> Yes, that is what I have assumed what happens. How do these libs go
>>> into an existing and working root-file-system with Debian Stretch?
>>
>> I'm a novice when it comes to vDSO, so someone please correct me if I'm 
>> wrong.
>>
>> From what I read vDSO is a library in the sense that it exports ELF 
>> symbols that applications and other libraries (libc in particular) can 
>> use, but it is not a file on disk.
> 
> Ah, ok. This would mean that the libc providing the gettimeofday()
> should be able to find out a modified changed vdso_data format by
> inspecting these ELF symbols.
>

I agree with Maarten here. There is a discovery mechanism in the libc based on
AT_SYSINFO_EHDR. This contains the address at which the vdso library is mapped
by the kernel to the userspace process.

>>
>> As such, which rootfs you use shouldn't matter, since the vDSO is not in 
>> the rootfs. Instead, it is contained in the kernel image. Searching for 
>> "linux-vdso.so.1" on packages.debian.org indeed returns no hits.
>>
>> There is a check in arch/mips/vdso/Makefile that disables vDSO on MIPS 
>> when building the kernel with binutils < 2.25. I don't know if that is 
>> in any way related to this issue.
> 
> What still does not fit into the picture is the errno = 1 i.e. EPERM.
> Maybe I have to study the libc code that tries to read the ELF symbols
> you have mentioned. It may fail for unknown reasons.
> 

This is what I was going to suggest next. It might be that something is not
working there.

Let us know your findings.

> BR and thanks,
> Nikolaus
> 

-- 
Regards,
Vincenzo

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