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    SubjectRe: why the decompressed procedure move kernel from address 0x100000(1M) to 0x1000000(16M) +x
    On 06/03/2012 01:41 AM, Eric W. Biederman wrote:
    >
    > Time wise copying the kernel probably takes a millisecond or less on
    > modern hardware, so I don't think it is a particularly large concern.
    >

    If your boot time budget is a second, getting rid of a millisecond helps.

    > Looking at parse_elf I see two misfeatures.
    > - parse_elf is short some memsets for the ELF sections that are larger
    > in memory than they are in the file data.
    > - We don't return the entry point of the elf header and instead hard
    > code the beginning of the file data.
    >
    > The oddest thing about parse_elf is what makes the copies parse_elf
    > performs safe. It just happens to be the case that because of the
    > way ld lays out the file those copies turn into a single memmove
    > that just strips off the elf header and program header.
    >
    > So it should be trivial to and perhaps even safer to decompress the
    > program segments to their final destination.
    >
    > Looking at the way the code is evolving, I suspect we should just give
    > up overlapping compressed data and uncompressed data. The elf header
    > logic in theory allows the code in a more arbitrary order, and it
    > doesn't look like anyone has done a worst case space analysis for
    > anything except gzip. The code works most of the time today but
    > I do wonder if it is safe.
    >
    > Additionally at the rate we are adding compression algorithms I don't
    > believe that all of the compression alorigthms use the gzip footer that
    > reports the uncompressed length of the file. So I suspect it would be
    > wise to get z_output_len from simply examining the uncompressed file
    > that we feed to compression programs, aka vmlinux.bin.all-y
    >
    > Perhaps I am wrong but I have the strongest feeling we are playing with
    > fire and getting very lucky right now.

    I think it might be even worse; on x86-64 we produce 2 MiB of completely
    useless zero-padding after the header. I have wanted to get rid of it
    but I'm not sure if the ELF code is robust enough. The code is a
    hackwork and yes, should be replaced by decompressing into the
    designated target areas.

    -hpa

    --
    H. Peter Anvin, Intel Open Source Technology Center
    I work for Intel. I don't speak on their behalf.



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