Re: [PATCH 3/3] makedumpfile: Rewrite readpage_elf

[Ivan Delalande <colona@arista.com>]

On Wed, Feb 10, 2016 at 08:50:09AM +0100, Petr Tesarik wrote:
> The current code in readpage_elf (and readpage_elf_parallel) is extremely
> hard to follow. Additionally, it still does not cover all possible cases.
> For example, attempts to read outside of any ELF segment will end up with
> phys_start being 0, frac_head a negative number, interpreted as a large
> positive number by memset() and write past buffer end.
> 
> Instead of trying to handle even more "corner cases", I rewrote the
> algorithm from scratch. The basic idea is simple: set a goal to fill the
> page buffer with data, then work towards that goal by:
> 
>   - filling holes with zeroes (see Note below),
>   - p_filesz portions with file data and
>   - remaining p_memsz portions again with zeroes.
> 
> Repeat this method for each LOAD until the goal is achieved, or an error
> occurs. In most cases, the loop runs only once.
> 
> Note: A "hole" is data at a physical address that is not covered by any
> ELF LOAD program header. In other words, the ELF file does not specify
> any data for such a hole (not even zeroes). So, why does makedumpfile
> fill them with zeroes? It's because makedumpfile works with page
> granularity (the compressed format does not even have a way to store
> a partial page), so if only part of a page is stored, a complete page
> must be provided to make this partial data accessible.
> 
> Credits to Ivan Delalande <colona@arista.com> who first found the
> problem and wrote the original fix.
> 
> Signed-off-by: Petr Tesarik <ptesarik@suse.com>

Tested-by: Ivan Delalande <colona@arista.com>

Dump-dmesg works well and gives the expected results with our various
setups (x86_64 only). Thanks for your work Petr!

-- 
Ivan "Colona" Delalande
Arista Networks

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