Re: Improving asan/ubsan support

On Fri Dec 1, 2023 at 3:00 AM CST, Alexander Lakhin wrote:
> [ this thread separated from [1] as the discussion focus shifted ]
>
> H Andres,
>
> 29.11.2023 22:39, Andres Freund wrote:
> >> I use the following:
> >> ASAN_OPTIONS=detect_leaks=0:abort_on_error=1:print_stacktrace=1:\
> >> disable_coredump=0:strict_string_checks=1:check_initialization_order=1:\
> >> strict_init_order=1:detect_stack_use_after_return=0
> > I wonder if we should add some of these options by default ourselves. We could
> > e.g. add something like the __ubsan_default_options() in
> > src/backend/main/main.c to src/port/... instead, and return a combination of
> > "our" options (like detect_leaks=0) and the ones from the environment.
>
> I think that such explicit expression of the project policy regarding
> sanitizer checks is for good, but I see some obstacles on this way.
>
> First, I'm not sure what to do with new useful options/maybe new option
> values, that will appear in sanitizers eventually. Should the only options,
> that are supported by all sanitizers' versions, be specified, or we may
> expect that unsupported options/values would be ignored by old versions?
>
> Second, what to do with other binaries, that need detect_leaks=0, for
> example, that same ecpg?
>
> > ISTM that, if it actually works as I theorize it should, using
> > __attribute__((no_sanitize("address"))) would be the easiest approach
> > here. Something like
> >
> > #if defined(__has_feature) && __has_feature(address_sanitizer)
> > #define pg_attribute_no_asan __attribute__((no_sanitize("address")))
> > #else
> > #define pg_attribute_no_asan
> > #endif
> >
> > or such should work.
>
> I've tried adding:
>  bool
> +__attribute__((no_sanitize("address")))
>  stack_is_too_deep(void)
>
> and it does work got me with clang 15, 18: `make check-world` passes with
> ASAN_OPTIONS=detect_leaks=0:abort_on_error=1:print_stacktrace=1:\
> disable_coredump=0:strict_string_checks=1:check_initialization_order=1:\
> strict_init_order=1:detect_stack_use_after_return=1
> UBSAN_OPTIONS=abort_on_error=1:print_stacktrace=1
>
> (with a fix for pg_bsd_indent applied [2])
>
> But with gcc 11, 12, 13 I get an assertion failure during `make check`:
> #4  0x00007fabadcd67f3 in __GI_abort () at ./stdlib/abort.c:79
> #5  0x0000557f35260382 in ExceptionalCondition (conditionName=0x557f35ca51a0 "(uintptr_t) buffer ==
> TYPEALIGN(PG_IO_ALIGN_SIZE, buffer)", fileName=0x557f35ca4fc0 "md.c", lineNumber=471) at assert.c:66
> #6  0x0000557f34a3b2bc in mdextend (reln=0x6250000375c8, forknum=MAIN_FORKNUM, blocknum=18, buffer=0x7fabaa800020,
> skipFsync=true) at md.c:471
> #7  0x0000557f34a45a6f in smgrextend (reln=0x6250000375c8, forknum=MAIN_FORKNUM, blocknum=18, buffer=0x7fabaa800020,
> skipFsync=true) at smgr.c:501
> #8  0x0000557f349139ed in RelationCopyStorageUsingBuffer (srclocator=..., dstlocator=..., forkNum=MAIN_FORKNUM,
> permanent=true) at bufmgr.c:4386
>
> The buffer (buf) declared as follows:
> static void
> RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
>                                RelFileLocator dstlocator,
>                                ForkNumber forkNum, bool permanent)
> {
> ...
>     PGIOAlignedBlock buf;
> ...
>
> But as we can see, the buffer address is really not 4k-aligned, and that
> offset 0x20 added in run-time only when the server started with
> detect_stack_use_after_return=1.
> So it looks like the asan feature detect_stack_use_after_return implemented
> in gcc allows itself to add some data on stack, that breaks our alignment
> expectations. With all three such Asserts in md.c removed,
> `make check-world` passes for me.

Decided to do some digging into this, and Google actually documents[0]
how it works. After reading the algorithm, it is obvious why this fails.
What happens if you throw an __attribute__((no_sanitize("address")) on
the function? I assume the Asserts would then pass. The commit[1] which
added pg_attribute_aligned() provides insight as to why the Asserts
exist.

> /* If this build supports direct I/O, the buffer must be I/O aligned. */

Disabling instrumentation in functions which use this specific type when
the build supports direct IO seems like the best solution.

> > One thing that's been holding me back on trying to do something around this is
> > the basically non-existing documentation around all of this. I haven't even
> > found documentation referencing the fact that there are headers like
> > sanitizer/asan_interface.h, you just have to figure that out yourself. Compare
> > that to something like valgrind, which has documented this at least somewhat.
>
> Yes, so maybe it's reasonable to support only basic/common features (such
> as detect_leaks), leaving advanced ones for ad-hoc usage till they prove
> their worthiness.

Possibly, but I think I would rather see upstream support running with
all features with instrumentation turned off in various sections of
code. Even some assistance from AddressSanitizer is better than none.
Here[1][2] are all the AddressSanitizer flags for those curious.

> Best regards,
> Alexander
>
> [1] https://www.postgresql.org/message-id/flat/CWTLB2WWVJJ2.2YV6ERNOL1WVF%40neon.tech
> [2] https://www.postgresql.org/message-id/591971ce-25c1-90f3-0526-5f54e3ebb32e%40gmail.com

I personally would like to see Postgres have support for
AddressSanitizer. I think it already supports UndefinedBehaviorSanitizer
if I am remembering the buildfarm properly. AddressSanitizer has been so
helpful in past experiences writing C.

[0]: https://github.com/google/sanitizers/wiki/AddressSanitizerUseAfterReturn#algorithm
[1]: https://github.com/postgres/postgres/commit/faeedbcefd40bfdf314e048c425b6d9208896d90
[2]: https://github.com/google/sanitizers/wiki/AddressSanitizerFlags
[3]: https://github.com/google/sanitizers/wiki/SanitizerCommonFlags

--
Tristan Partin
Neon (https://neon.tech)