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journal: deal better with reading from zeroed out journal mmaps #15557

Merged
merged 9 commits into from May 21, 2020
Merged

journal: deal better with reading from zeroed out journal mmaps #15557

merged 9 commits into from May 21, 2020

Commits on Apr 23, 2020

  1. macro: add READ_NOW() macro for force reading of memory, making a copy 

    When accessing journal files we generally are fine when values change
beneath our feet, while we are looking at them, as long as they change
from something valid to zero. This is required since we nowadays
forcibly unallocate journal files on vacuuming, to ensure they are
actually released.

However, we need to make sure that the validity checks we enforce are
done on suitable copies of the fields in the file. Thus provide a macro
that forces a copy, and disallows the compiler from merging our copy
with the actually memory where it is from.
    @poettering
    poettering committed Apr 23, 2020
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  2. journal-file: avoid risky subtraction when validity checking object 

    The value might change beneath what we do, and hence let's avoid any
chance of underflow.
    @poettering
    poettering committed Apr 23, 2020
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  3. journal: use a bitfield where appropriate

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    poettering committed Apr 23, 2020
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  4. journal: use structured initialization for Location structure

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  5. memory-util: add missing () in macro evaulation

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    poettering committed Apr 23, 2020
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  6. journal: don't assert on mmap'ed object type 

    Mappings canbe replaced by all zeroes under our feet if vacuuming
decides to unallocate some file. Hence let's not check for this kind of
stuff in an assert.

(Typically, we should genreate runtime errors in this case, in
particular EBADMSG, which the callers generally look for. But in this
case this is just an extra precaution check anyway, so let's just remove
it.)
    @poettering
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  7. journal: several minor coding style fixes/clean-ups

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  8. journal: make sure to explicitly copy out values of mmap before doing… 

    … arithmetics on them

Our journal code is generally supposed to be written in a fashion that
the underlying file can be deallocated any time, i.e. our mmap of it
suddenly becomes all zeroes. The idea is that we catch that when parsing
everything. For that to work safely we need to make sure that when doing
arithmetics or comparisons on values read from the map we don't run into
TTOCTTOU issues when determining validity. Hence we need to copy out the
values before use and operate on the copies. This requires some special
care since the C compiler could suppress our copies as optimization.
Hence use the new READ_NOW() macro to force a copy by using memcpy(),
and use it whenever we start doing an arithmetic operation on it, or
validity checking of multiple steps.

Fixes: systemd#14943
    @poettering
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  9. journal: no need to check offset twice, journal_file_move_to_object()… 

    … does it again
    @poettering
    poettering committed Apr 23, 2020
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