Bug 256211 - valgrind shows one leak every time
Summary: valgrind shows one leak every time
Status: New
Alias: None
Product: Base System
Classification: Unclassified
Component: kern (show other bugs)
Version: 13.0-RELEASE
Hardware: amd64 Any
: --- Affects Some People
Assignee: freebsd-bugs (Nobody)
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2021-05-28 02:49 UTC by mazoti
Modified: 2021-06-01 06:52 UTC (History)
2 users (show)

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Description mazoti 2021-05-28 02:49:49 UTC
I develop a very small garbage collector in C and doesn't matter how many malloc()s I did it always suppressed 4096 bytes in 1 block (1 free() missed) 

git clone https://github.com/mazoti/cgc
cd cgc
clang -g example.c
valgrind ./a.out

I tested exact the same code in debian linux and everything is fine (no leaks). I don't know if it is a libc, kernel or valgrind bug.


Best regards,
Comment 1 Paul Floyd 2021-06-01 06:52:17 UTC
There is no problem here. If you run Valgrind with -s you will get

--4534-- used_suppression:      1 MEMCHECK-LIBC-REACHABLE /usr/local/libexec/valgrind/default.supp:589 suppressed: 4,096 bytes in 1 blocks

That means that the 4k still in use is known and suppressed by the system suppressions file

If you run Valgrind with --defaut-suppressions=no  then you will see

==4537== 4,096 bytes in 1 blocks are still reachable in loss record 1 of 1
==4537==    at 0x484C8A4: malloc (in /usr/local/libexec/valgrind/vgpreload_memcheck-amd64-freebsd.so)
==4537==    by 0x4974AA3: ??? (in /lib/libc.so.7)
==4537==    by 0x4987278: ??? (in /lib/libc.so.7)
==4537==    by 0x497B012: ??? (in /lib/libc.so.7)
==4537==    by 0x497AD89: vfprintf_l (in /lib/libc.so.7)
==4537==    by 0x4975AF3: printf (in /lib/libc.so.7)
==4537==    by 0x201D48: main (example.c:16)

In theory it would be possible to remove this suppression by modifying libc. On Linux, GNU libc has a __libc_freeres function, In normal use, Linux GNU libc applications do not call this function. Valgrind does some fiddling on guest application termination and _does_ call thisa function. The advantages of using freeres over suppressions are that
a) it is slighlty cleaner
b) it is less sensitive (suppressions tend to change over time and libc could add more uses of reachable memory).

If this really bothers you, open a bugzilla item for libc to add a freeres function and then I can add that to Valgrind.