Re: Refactoring backend fork+exec code

On Fri Dec 1, 2023 at 2:44 PM CST, Heikki Linnakangas wrote:
> On 01/12/2023 16:00, Alexander Lakhin wrote:
> > Maybe you could look at issues with running `make check` under Valgrind
> > when server built with CPPFLAGS="-DUSE_VALGRIND -DEXEC_BACKEND":
> > # +++ regress check in src/test/regress +++
> > # initializing database system by copying initdb template
> > # postmaster failed, examine ".../src/test/regress/log/postmaster.log" for the reason
> > Bail out!make[1]: ***
> >
> > ...
> > 2023-12-01 16:48:39.136 MSK postmaster[1307988] LOG:  listening on Unix socket
"/tmp/pg_regress-pPFNk0/.s.PGSQL.55312"
> > ==00:00:00:01.692 1259396== Syscall param write(buf) points to uninitialised byte(s)
> > ==00:00:00:01.692 1259396==    at 0x5245A37: write (write.c:26)
> > ==00:00:00:01.692 1259396==    by 0x51BBF6C: _IO_file_write@@GLIBC_2.2.5 (fileops.c:1180)
> > ==00:00:00:01.692 1259396==    by 0x51BC84F: new_do_write (fileops.c:448)
> > ==00:00:00:01.692 1259396==    by 0x51BC84F: _IO_new_file_xsputn (fileops.c:1254)
> > ==00:00:00:01.692 1259396==    by 0x51BC84F: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1196)
> > ==00:00:00:01.692 1259396==    by 0x51B1056: fwrite (iofwrite.c:39)
> > ==00:00:00:01.692 1259396==    by 0x552E21: internal_forkexec (postmaster.c:4518)
> > ==00:00:00:01.692 1259396==    by 0x5546A1: postmaster_forkexec (postmaster.c:4444)
> > ==00:00:00:01.692 1259396==    by 0x55471C: StartChildProcess (postmaster.c:5275)
> > ==00:00:00:01.692 1259396==    by 0x557B61: PostmasterMain (postmaster.c:1454)
> > ==00:00:00:01.692 1259396==    by 0x472136: main (main.c:198)
> > ==00:00:00:01.692 1259396==  Address 0x1ffeffdc11 is on thread 1's stack
> > ==00:00:00:01.692 1259396==  in frame #4, created by internal_forkexec (postmaster.c:4482)
> > ==00:00:00:01.692 1259396==
> >
> > With memset(param, 0, sizeof(*param)); added at the beginning of
> > save_backend_variables(), server starts successfully, but then
> > `make check` fails with other Valgrind error.
>
> That's actually a pre-existing issue, I'm seeing that even on unpatched
> 'master'.
>
> In a nutshell, the problem is that the uninitialized padding bytes in
> BackendParameters are written to the file. When we read the file back,
> we don't access the padding bytes, so that's harmless. But Valgrind
> doesn't know that.
>
> On Windows, the file is created with
> CreateFileMapping(INVALID_HANDLE_VALUE, ...) and we write the variables
> directly to the mapping. If I understand the Windows API docs correctly,
> it is guaranteed to be initialized to zeros, so we don't have this
> problem on Windows, only on other platforms with EXEC_BACKEND. I think
> it makes sense to clear the memory on other platforms too, since that's
> what we do on Windows.
>
> Committed a fix with memset(). I'm not sure what our policy with
> backpatching this kind of issues is. This goes back to all supported
> versions, but given the lack of complaints, I chose to not backpatch.

Seems like a harmless think to backpatch. It is conceivable that someone
might want to run Valgrind on something other than HEAD.

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