Обсуждение: How to test Postgres for any unaligned memory accesses?

Hi,

I'm trying to test Postgres code for any unaligned memory accesses. I
used a hack shown at [1] and put it in exec_simple_query, then I'm
seeing a SIGBUS error from SplitIdentifierString's strncpy, see [2].
It looks like the SIGBUS error occurs even if a simple memcpy(for
testing purpose) is done in recomputeNamespacePath or
SplitIdentifierString.

I'm not sure this is the right way. I would like to know whether there
is a standard way of testing Postgres code for any unaligned memory
accesses. Thanks. Any help would be appreciated.

[1] - https://www.programmersought.com/article/17701994124/
+/* Enable Alignment Checking */
+#if defined(__GNUC__)
+# if defined(__i386__)
+    /* Enable Alignment Checking on x86 */
+    __asm__("pushf\norl $0x40000,(%esp)\npopf");
+# elif defined(__x86_64__)
+     /* Enable Alignment Checking on x86_64 */
+    __asm__("pushf\norl $0x40000,(%rsp)\npopf");
+# endif
+#endif

[2]
Program received signal SIGBUS, Bus error.
0x00007f5067188d36 in __strncpy_sse2_unaligned () from /lib64/libc.so.6
(gdb) bt
#0  0x00007f5067188d36 in __strncpy_sse2_unaligned () from /lib64/libc.so.6
#1  0x0000000000ada740 in SplitIdentifierString (rawstring=0x1146620 "\"$user",
    separator=44 ',', namelist=0x7ffcdf1911d0) at varlena.c:3817
#2  0x00000000005d203b in recomputeNamespacePath () at namespace.c:3761
#3  0x00000000005cde11 in FuncnameGetCandidates (names=0x1145e08,
nargs=2, argnames=0x0,
    expand_variadic=true, expand_defaults=true, missing_ok=false) at
namespace.c:971
#4  0x0000000000647dcb in func_get_detail (funcname=0x1145e08, fargs=0x1146570,
    fargnames=0x0, nargs=2, argtypes=0x7ffcdf191540, expand_variadic=true,
    expand_defaults=true, funcid=0x7ffcdf1916d8, rettype=0x7ffcdf1916dc,
    retset=0x7ffcdf19152f, nvargs=0x7ffcdf191528, vatype=0x7ffcdf191524,
    true_typeids=0x7ffcdf191538, argdefaults=0x7ffcdf191530) at
parse_func.c:1421
#5  0x0000000000645961 in ParseFuncOrColumn (pstate=0x11462e8,
funcname=0x1145e08,
    fargs=0x1146570, last_srf=0x0, fn=0x1145f28, proc_call=false, location=14)
    at parse_func.c:265

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com

Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com> writes:
> I'm trying to test Postgres code for any unaligned memory accesses. I
> used a hack shown at [1] and put it in exec_simple_query, then I'm
> seeing a SIGBUS error from SplitIdentifierString's strncpy, see [2].

Regardless of Postgres' policy about alignment safety, glibc sees
no reason to avoid unaligned accesses on x86 hardware.  If you want
to test this sort of thing on hardware that's not actually alignment
picky, you have to enlist the toolchain's help.

> I'm not sure this is the right way. I would like to know whether there
> is a standard way of testing Postgres code for any unaligned memory
> accesses. Thanks. Any help would be appreciated.

Per c.h, late-model compilers have options for this:

 * Testing can be done with "-fsanitize=alignment -fsanitize-trap=alignment"
 * on clang, or "-fsanitize=alignment -fno-sanitize-recover=alignment" on gcc.

We have at least one buildfarm member using the former.  I have no idea
how water-tight these checks are though.  They don't seem to cause very
much slowdown, which is suspicious :-(

            regards, tom lane

On Fri, Apr 23, 2021 at 7:25 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > I'm not sure this is the right way. I would like to know whether there
> > is a standard way of testing Postgres code for any unaligned memory
> > accesses. Thanks. Any help would be appreciated.
>
> Per c.h, late-model compilers have options for this:
>
>  * Testing can be done with "-fsanitize=alignment -fsanitize-trap=alignment"
>  * on clang, or "-fsanitize=alignment -fno-sanitize-recover=alignment" on gcc.

Thanks Tom!

I used the above gcc compiler flags to see if they catch memory
alignment issues. The way I tested on my dev system (x86_64 platform
with Ubuntu OS)  was that I commented out max aligning specialSize in
PageInit, compiled the source code with and without the alignment
flags. make check failed with the alignment checking flags, it passed
without the flags.

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com