Re: [musl] The heap memory performance (malloc/free/realloc) is significantly degraded in musl 1.2 (compared to 1.1)

[Gabriel Ravier <gabravier@gmail.com>]

On 9/19/22 20:14, Szabolcs Nagy wrote:
> * baiyang <baiyang@gmail.com> [2022-09-20 01:40:48 +0800]:
>> I looked at the code of tcmalloc, but I didn't find any of the problems you mentioned in the implementation of malloc_usable_size (see: https://github.com/google/tcmalloc/blob/9179bb884848c30616667ba129bcf9afee114c32/tcmalloc/tcmalloc.cc#L1099 ).
>>
>> On the contrary, similar to musl, tcmalloc also directly uses the return value of malloc_usable_size in its realloc implementation to determine whether memory needs to be reallocated: https://github.com/google/tcmalloc/blob/9179bb884848c30616667ba129bcf9afee114c32/tcmalloc/tcmalloc.cc#L1499
>>
>> I think this is enough to show that the return value of malloc_usable_size in tcmalloc is accurate and reliable, otherwise its own realloc will cause a segment fault.
> obviously internally the implementation can use the internal chunk size...
>
> GetSize(p) is not the exact size (that the user allocated) but an internal
> size (which may be larger) and that must not be exposed *outside* of the
> malloc implementation (other than for diagnostic purposes).
>
> you can have 2 views:
>
> (1) tcmalloc and jemalloc are buggy because they expose an internal
>      that must not be exposed (becaues it can break user code).
>
> (2) user code is buggy if it uses malloc_usable_size for any purpose
>      other than diagnostic/statistics (because other uses are broken
>      on many implementations).
>
> either way the brokenness you want to support is a security hazard
> and you are lucky that musl saves the day: it works hard not to
> expose internal sizes so the code you seem to care about can operate
> safely (which is not true on tcmalloc and jemalloc: the compiler
> may break that code).

While I would agree that using malloc_usable_size is generally not a 
great idea (it's at most acceptable as a small micro-optimization, but I 
would only ever expect it to be seen in very well-tested code in very 
hot loops, as it is indeed quite easily misused), it seems like a bit of 
a stretch to say that all of:

- sqlite3 (https://github.com/sqlite/sqlite/blob/master/src/mem1.c)

- systemd 
(https://github.com/systemd/systemd/blob/main/src/basic/alloc-util.h , 
along with all files using MALLOC_SIZEOF_SAFE, i.e. 
src/basic/alloc-util.c, src/basic/compress.c, src/basic/fileio.c, 
src/basic/memory-util.h, src/basic/recurse-dir.c, 
src/basic/string-util.c, src/libsystemd/sd-netlink/netlink-socket.c, 
src/shared/journal-importer.c, src/shared/varlink.c, 
src/test/test-alloc-util.c and src/test/test-compress.c)

- rocksdb 
(https://github.com/facebook/rocksdb/blob/main/table/block_based/filter_policy.cc 
, along with at least 20 other uses)

- folly (https://github.com/facebook/folly/blob/main/folly/small_vector.h)

- lzham_codec 
(https://github.com/richgel999/lzham_codec/blob/master/lzhamdecomp/lzham_mem.cpp)

- quickjs 
(https://raw.githubusercontent.com/bellard/quickjs/master/quickjs.c)

- redis (https://github.com/redis/redis/blob/unstable/src/networking.c, 
along with a few other uses elsewhere)

along with so many more well-known projects that I've given up on 
listing them, are all buggy because of their usage of malloc_usable_size...