Re: kdump in upstream kexec-tools

[Simon Horman]

On Wed, Apr 18, 2018 at 07:28:54PM +0100, Russell King wrote:
> On Wed, Apr 18, 2018 at 03:01:08PM +0200, Simon Horman wrote:
> > On Tue, Apr 17, 2018 at 10:01:13AM +0100, Russell King wrote:
> > > On Tue, Apr 17, 2018 at 10:20:08AM +0530, Bhupesh Sharma wrote:
> > > > Hi,
> > > > 
> > > > I was working on improving documentation/structure of the upstream
> > > > kexec-tools and I was wondering what is the purpose of the 'kdump'
> > > > directory inside the kexec-tools.
> > > > 
> > > > This kdump utility seems to cause confusion with the 'kdump' utility
> > > > present inside some distributions (for e.g.  '/usr/sbin/kdump' present
> > > > inside fedora) due to the same naming convention and so we should
> > > > populate/modify the kdump man page to indicate the same, so as to
> > > > avoid confusion.
> > > > 
> > > > Presently here are the contents of this directory:
> > > > 
> > > > # ls kdump/
> > > > kdump.8  kdump.c  Makefile
> > > > 
> > > > - Out of these the kdump man documentation (kdump.8) is just a
> > > > placeholder as suggested by the man page documentation: "kdump - This
> > > > is just a placeholder until real man page has been written"
> > > > 
> > > > - Looking at kdump.c :
> > > > 
> > > > 1. I understand that this code is mainly used to read a crashdump from
> > > > memory. One can run the same using:
> > > > 
> > > > # kdump 
> > > > 
> > > > where start_addr is basically the start address of the core dump
> > > > (which can also be represented via the 'elfcorehdr' environment
> > > > variable being passed to the crash kernel which represents the
> > > > physical address of the start of the ELF header)
> > > > 
> > > > 2. This tool needs READ_ONLY access to /dev/mem (so we need to set
> > > > CONFIG_STRICT_DEVMEM configuration option accordingly).
> > > > 
> > > > 3. The code thereafter reads (via mmap) and verifies the ELF header.
> > > > Subsequently it reads (via mmap) the program header.
> > > > 
> > > > 4. Then we collect all the notes and write on STDOUT all the headers
> > > > and notes found in the crashdump collected from memory.
> > > > 
> > > > So, as per my understanding even in absence of (more powerful) tools
> > > > like crash (or gdb), we can still go ahead and read the crashdump from
> > > > memory and display all the headers and notes present in the same on
> > > > standard serial out interface using this kdump utility.
> > > > 
> > > > This is probably a good to have feature for systems which have very
> > > > simple/minimal rootfs (and I see that a few arm32 systems seem to use
> > > > the same as well) or are low on memory availability.
> > > > 
> > > > Now, I wanted to confirm if the 'kdump' utility for reading crashdump
> > > > collected from memory is still needed (as the last commit is dated
> > > > back to 2016 and was done for arm32 systems). If yes, I can go ahead
> > > > and enhance the kdump man page to include the description given above
> > > > - so that it helps users understand how to run the tool.
> > > > 
> > > > Please share your opinions.
> > > 
> > > Firstly, please use:
> > > 
> > >   git://git.armlinux.org.uk/~rmk/kexec-tools.git
> > > 
> > > for ARM systems - this has some important fixes that aren't in the
> > > mainline repository.
> > 
> > I apologise if this is due to omission on my part,
> > can we work towards getting them in the mainline repository?
> 
> The problem on ARM was caused because you applied the wrong version of
> the patches I sent out.  When I noticed and reported it, there was no
> response.
> 
> My only option over that intervening six months is to provide people
> with something that actually works properly on 32-bit ARM has been
> to publish my own kexec-tools git tree with the appropriate fixes in.
> 
> It now contains a couple more patches than just fixing that up.
> 
> If you want to merge from the above URL, be my guest, but as far as
> I'm concerned, it's been proven that sending patches for merging is
> open to mistakes happening.  That wouldn't be too bad if it hadn't
> taken more than six months to get your attention on this issue.

Thanks, pulled.

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Re: kdump in upstream kexec-tools

[Russell King]

On Wed, Apr 18, 2018 at 03:01:08PM +0200, Simon Horman wrote:
> On Tue, Apr 17, 2018 at 10:01:13AM +0100, Russell King wrote:
> > On Tue, Apr 17, 2018 at 10:20:08AM +0530, Bhupesh Sharma wrote:
> > > Hi,
> > > 
> > > I was working on improving documentation/structure of the upstream
> > > kexec-tools and I was wondering what is the purpose of the 'kdump'
> > > directory inside the kexec-tools.
> > > 
> > > This kdump utility seems to cause confusion with the 'kdump' utility
> > > present inside some distributions (for e.g.  '/usr/sbin/kdump' present
> > > inside fedora) due to the same naming convention and so we should
> > > populate/modify the kdump man page to indicate the same, so as to
> > > avoid confusion.
> > > 
> > > Presently here are the contents of this directory:
> > > 
> > > # ls kdump/
> > > kdump.8  kdump.c  Makefile
> > > 
> > > - Out of these the kdump man documentation (kdump.8) is just a
> > > placeholder as suggested by the man page documentation: "kdump - This
> > > is just a placeholder until real man page has been written"
> > > 
> > > - Looking at kdump.c :
> > > 
> > > 1. I understand that this code is mainly used to read a crashdump from
> > > memory. One can run the same using:
> > > 
> > > # kdump 
> > > 
> > > where start_addr is basically the start address of the core dump
> > > (which can also be represented via the 'elfcorehdr' environment
> > > variable being passed to the crash kernel which represents the
> > > physical address of the start of the ELF header)
> > > 
> > > 2. This tool needs READ_ONLY access to /dev/mem (so we need to set
> > > CONFIG_STRICT_DEVMEM configuration option accordingly).
> > > 
> > > 3. The code thereafter reads (via mmap) and verifies the ELF header.
> > > Subsequently it reads (via mmap) the program header.
> > > 
> > > 4. Then we collect all the notes and write on STDOUT all the headers
> > > and notes found in the crashdump collected from memory.
> > > 
> > > So, as per my understanding even in absence of (more powerful) tools
> > > like crash (or gdb), we can still go ahead and read the crashdump from
> > > memory and display all the headers and notes present in the same on
> > > standard serial out interface using this kdump utility.
> > > 
> > > This is probably a good to have feature for systems which have very
> > > simple/minimal rootfs (and I see that a few arm32 systems seem to use
> > > the same as well) or are low on memory availability.
> > > 
> > > Now, I wanted to confirm if the 'kdump' utility for reading crashdump
> > > collected from memory is still needed (as the last commit is dated
> > > back to 2016 and was done for arm32 systems). If yes, I can go ahead
> > > and enhance the kdump man page to include the description given above
> > > - so that it helps users understand how to run the tool.
> > > 
> > > Please share your opinions.
> > 
> > Firstly, please use:
> > 
> >   git://git.armlinux.org.uk/~rmk/kexec-tools.git
> > 
> > for ARM systems - this has some important fixes that aren't in the
> > mainline repository.
> 
> I apologise if this is due to omission on my part,
> can we work towards getting them in the mainline repository?

The problem on ARM was caused because you applied the wrong version of
the patches I sent out.  When I noticed and reported it, there was no
response.

My only option over that intervening six months is to provide people
with something that actually works properly on 32-bit ARM has been
to publish my own kexec-tools git tree with the appropriate fixes in.

It now contains a couple more patches than just fixing that up.

If you want to merge from the above URL, be my guest, but as far as
I'm concerned, it's been proven that sending patches for merging is
open to mistakes happening.  That wouldn't be too bad if it hadn't
taken more than six months to get your attention on this issue.

-- 
Russell King

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Re: kdump in upstream kexec-tools

[Simon Horman]

On Tue, Apr 17, 2018 at 10:01:13AM +0100, Russell King wrote:
> On Tue, Apr 17, 2018 at 10:20:08AM +0530, Bhupesh Sharma wrote:
> > Hi,
> > 
> > I was working on improving documentation/structure of the upstream
> > kexec-tools and I was wondering what is the purpose of the 'kdump'
> > directory inside the kexec-tools.
> > 
> > This kdump utility seems to cause confusion with the 'kdump' utility
> > present inside some distributions (for e.g.  '/usr/sbin/kdump' present
> > inside fedora) due to the same naming convention and so we should
> > populate/modify the kdump man page to indicate the same, so as to
> > avoid confusion.
> > 
> > Presently here are the contents of this directory:
> > 
> > # ls kdump/
> > kdump.8  kdump.c  Makefile
> > 
> > - Out of these the kdump man documentation (kdump.8) is just a
> > placeholder as suggested by the man page documentation: "kdump - This
> > is just a placeholder until real man page has been written"
> > 
> > - Looking at kdump.c :
> > 
> > 1. I understand that this code is mainly used to read a crashdump from
> > memory. One can run the same using:
> > 
> > # kdump 
> > 
> > where start_addr is basically the start address of the core dump
> > (which can also be represented via the 'elfcorehdr' environment
> > variable being passed to the crash kernel which represents the
> > physical address of the start of the ELF header)
> > 
> > 2. This tool needs READ_ONLY access to /dev/mem (so we need to set
> > CONFIG_STRICT_DEVMEM configuration option accordingly).
> > 
> > 3. The code thereafter reads (via mmap) and verifies the ELF header.
> > Subsequently it reads (via mmap) the program header.
> > 
> > 4. Then we collect all the notes and write on STDOUT all the headers
> > and notes found in the crashdump collected from memory.
> > 
> > So, as per my understanding even in absence of (more powerful) tools
> > like crash (or gdb), we can still go ahead and read the crashdump from
> > memory and display all the headers and notes present in the same on
> > standard serial out interface using this kdump utility.
> > 
> > This is probably a good to have feature for systems which have very
> > simple/minimal rootfs (and I see that a few arm32 systems seem to use
> > the same as well) or are low on memory availability.
> > 
> > Now, I wanted to confirm if the 'kdump' utility for reading crashdump
> > collected from memory is still needed (as the last commit is dated
> > back to 2016 and was done for arm32 systems). If yes, I can go ahead
> > and enhance the kdump man page to include the description given above
> > - so that it helps users understand how to run the tool.
> > 
> > Please share your opinions.
> 
> Firstly, please use:
> 
>   git://git.armlinux.org.uk/~rmk/kexec-tools.git
> 
> for ARM systems - this has some important fixes that aren't in the
> mainline repository.

I apologise if this is due to omission on my part,
can we work towards getting them in the mainline repository?

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Re: kdump in upstream kexec-tools

[Bhupesh Sharma]

On Tue, Apr 17, 2018 at 6:21 PM, Russell King  wrote:
> On Tue, Apr 17, 2018 at 04:20:00PM +0530, Bhupesh Sharma wrote:
>> For e.g I use this tool on my arm64 board as follows:
>>
>> a. Read out the 'elfcorehdr' env variable passed to the crash kernel
>> and pass the same as an argument to the tool:
>>
>> Assuming that the 'elfcorehdr' spans the range ->
>> 0xffdf-0xffdf13ff, launch the tool as -
>>
>> # kdump
>> Cannot find the start of the core dump
>>
>> # kdump 0xffdf >> output_elf_file
>>
>> # file output_elf_file
>> output: ELF 64-bit LSB core file ARM aarch64, version 1 (SYSV)
>
> The contents should basically be the same (possibly with a different
> section ordering) as /proc/vmcore in the crashdump kernel.  If so,
> kdump serves no useful purpose, and ends up confusing the situation
> due to its inability to handle 32-bit ELF coredump files.
>
> It seems to me that the presence of /proc/vmcore obsoletes the kdump
> tool.

Fair enough. I will send a patch shortly to remove the obsolete kdump
tool from 'kexec-tools'.

Thanks,
Bhupesh

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Re: kdump in upstream kexec-tools

[Russell King]

On Tue, Apr 17, 2018 at 04:20:00PM +0530, Bhupesh Sharma wrote:
> For e.g I use this tool on my arm64 board as follows:
> 
> a. Read out the 'elfcorehdr' env variable passed to the crash kernel
> and pass the same as an argument to the tool:
> 
> Assuming that the 'elfcorehdr' spans the range ->
> 0xffdf-0xffdf13ff, launch the tool as -
> 
> # kdump
> Cannot find the start of the core dump
> 
> # kdump 0xffdf >> output_elf_file
> 
> # file output_elf_file
> output: ELF 64-bit LSB core file ARM aarch64, version 1 (SYSV)

The contents should basically be the same (possibly with a different
section ordering) as /proc/vmcore in the crashdump kernel.  If so,
kdump serves no useful purpose, and ends up confusing the situation
due to its inability to handle 32-bit ELF coredump files.

It seems to me that the presence of /proc/vmcore obsoletes the kdump
tool.

-- 
Russell King

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Re: kdump in upstream kexec-tools

[Russell King]

On Tue, Apr 17, 2018 at 10:20:08AM +0530, Bhupesh Sharma wrote:
> Hi,
> 
> I was working on improving documentation/structure of the upstream
> kexec-tools and I was wondering what is the purpose of the 'kdump'
> directory inside the kexec-tools.
> 
> This kdump utility seems to cause confusion with the 'kdump' utility
> present inside some distributions (for e.g.  '/usr/sbin/kdump' present
> inside fedora) due to the same naming convention and so we should
> populate/modify the kdump man page to indicate the same, so as to
> avoid confusion.
> 
> Presently here are the contents of this directory:
> 
> # ls kdump/
> kdump.8  kdump.c  Makefile
> 
> - Out of these the kdump man documentation (kdump.8) is just a
> placeholder as suggested by the man page documentation: "kdump - This
> is just a placeholder until real man page has been written"
> 
> - Looking at kdump.c :
> 
> 1. I understand that this code is mainly used to read a crashdump from
> memory. One can run the same using:
> 
> # kdump 
> 
> where start_addr is basically the start address of the core dump
> (which can also be represented via the 'elfcorehdr' environment
> variable being passed to the crash kernel which represents the
> physical address of the start of the ELF header)
> 
> 2. This tool needs READ_ONLY access to /dev/mem (so we need to set
> CONFIG_STRICT_DEVMEM configuration option accordingly).
> 
> 3. The code thereafter reads (via mmap) and verifies the ELF header.
> Subsequently it reads (via mmap) the program header.
> 
> 4. Then we collect all the notes and write on STDOUT all the headers
> and notes found in the crashdump collected from memory.
> 
> So, as per my understanding even in absence of (more powerful) tools
> like crash (or gdb), we can still go ahead and read the crashdump from
> memory and display all the headers and notes present in the same on
> standard serial out interface using this kdump utility.
> 
> This is probably a good to have feature for systems which have very
> simple/minimal rootfs (and I see that a few arm32 systems seem to use
> the same as well) or are low on memory availability.
> 
> Now, I wanted to confirm if the 'kdump' utility for reading crashdump
> collected from memory is still needed (as the last commit is dated
> back to 2016 and was done for arm32 systems). If yes, I can go ahead
> and enhance the kdump man page to include the description given above
> - so that it helps users understand how to run the tool.
> 
> Please share your opinions.

Firstly, please use:

  git://git.armlinux.org.uk/~rmk/kexec-tools.git

for ARM systems - this has some important fixes that aren't in the
mainline repository.

I think the kdump tool is dead.  It only supports the 64-bit ELF
format, and 32-bit ARM can either be 32-bit ELF or 64-bit ELF format
coredumps, depending whether LPAE is enabled.  I've asked questions
about this, and not got anywhere, so I now recommend not using that
tool.

Have you checked whether objcopy can copy the coredump from
/proc/vmcore to the filesystem?  That would permit saving of the
coredump image for later inspection by gdb.

-- 
Russell King

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