[LTP] [PATCH 2/2] bpf: Check truncation on 32bit div/mod by zero

[Petr Vorel]

Hi Richard,

> Add a test which checks for a number of issues surrounding division by
> zero.

> +++ b/testcases/kernel/syscalls/bpf/bpf_prog05.c
...
> +/*\
> + * [Description]
> + *
> + * Compare the effects of 32-bit div/mod by zero with the "expected"
FYI this causes docparser error:

/usr/src/ltp/docparse/testinfo.pl metadata.json
, or ] expected while parsing array, at character offset 70801 (before "expected"",\n     "b...") at /usr/src/ltp/docparse/testinfo.pl line 423

Because we don't escap double quotes (among other issues previously reported,
e.g. tabs), it produces invalid json:

   "doc": [
     "[Description]",
     "",
     "Compare the effects of 32-bit div/mod by zero with the "expected"",
     "behaviour.",
     "",

But I tested it on various OS and the code compiles and runs ok.

I also looked at the code (very interesting), but with my minimal bpf knowledge
I'm not the one who should review it.

> + * behaviour.
> + *
> + * The commit "bpf: fix subprog verifier bypass by div/mod by 0
> + * exception", changed div/mod by zero from exiting the current
> + * program to setting the destination register to zero (div) or
> + * leaving it untouched (mod).
> + *
> + * This solved one verfier bug which allowed dodgy pointer values, but
                      ^
                      verifier

> + * it turned out that the source register was being 32-bit truncated
> + * when it should not be. Also the destination register for mod was
> + * not being truncated when it should be.
> + *
> + * So then we have the following two fixes:
> + * "bpf: Fix 32 bit src register truncation on div/mod"
> + * "bpf: Fix truncation handling for mod32 dst reg wrt zero"
- * "bpf: Fix 32 bit src register truncation on div/mod"
- * "bpf: Fix truncation handling for mod32 dst reg wrt zero"
(making it list)

> + *
> + * Testing for all of these issues is a problem. Not least because
> + * division by zero is undefined, so in theory any result is
> + * acceptable so long as the verifier and runtime behaviour
> + * match.
> + *
> + * However to keep things simple we just check if the source and
> + * destination register runtime values match the current upstream
> + * behaviour at the time of writing.
> + *
> + * If the test fails you may have one or more of the above patches
> + * missing. In this case it is possible that you are not vulnerable
> + * depending on what other backports and fixes have been applied. If
> + * upstream changes the behaviour of division by zero, then the test
> + * will need updating.
> +\*/
@metan: remove this before merge.


Kind regards,
Petr