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Subject: Re: MIPS: bug: gettimeofday syscall broken on CI20 board
From:   "H. Nikolaus Schaller" <hns@goldelico.com>
In-Reply-To: <4807842.gtHLO0kk0V@hyperion>
Date:   Thu, 28 Nov 2019 14:48:46 +0100
Cc:     mips-creator-ci20-dev@googlegroups.com,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Ralf Baechle <ralf@linux-mips.org>,
        Paul Burton <paul.burton@mips.com>, linux-mips@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Discussions about the Letux Kernel 
        <letux-kernel@openphoenux.org>
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References: <18788C50-F29B-4BD7-89F6-B056FF490214@goldelico.com> <7b6275c7-ab2b-a647-6bf7-d5e1c4523c98@arm.com> <D1CE4D1E-9A42-4FAE-90A9-615C38B979C0@goldelico.com> <4807842.gtHLO0kk0V@hyperion>
To:     Maarten ter Huurne <maarten@treewalker.org>
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> Am 28.11.2019 um 14:29 schrieb Maarten ter Huurne <maarten@treewalker.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@arm.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.

> 
> 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.

BR and thanks,
Nikolaus