Date: Mon, 7 May 2018 17:27:03 -0400 From: Phil Shafer <phil@juniper.net> To: <freebsd-arch@freebsd.org> Subject: initialization problem w/ thread-specific .tbss data on i386 Message-ID: <201805072127.w47LR3W5060281@idle.juniper.net>
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I have a problem reported with libxo-based applications running
under FreeBSD-11-stable on i386 boxes that I think is related
to rtld:
When I breakpoint on main() and dump the contents of my uninitialized
thread-specific variable, it has not been initialized to zeroes.
I don't see this problem on 64-bit systems, only on i386 ones.
When I look at the rtld code, it appears to memset the .tbss to
zero (/usr/src/libexec/rtld-elf/rtld.c:allocate_tls) in the
non-arch-specific code so the arch shouldn't matter, but something
is not working right.
So I'm looking for a helpful clue, such as how to debug rtld to see
why this isn't being zeroed. I thought I'd use:
gdb /libexec/ld-elf.so.1
run /usr/bin/uptime
for this doesn't work for me (SEGV with a callstack that doesn't
make sense).
For this instance, the work around is to initialize the contents
of xo_default_handle to zero so it's not in the .tbss, but I'd like
to understand what's failing. In truth, I just have a hard time
blaming rtld, even though this is issue is an obscure intersection
of weird things (.tbbs on i386). Perhaps it's something wrong with
how the library is built or similar. But given that it's not zeroed
when main() get control, something's clearly broken.
Details follow:
I declare my variable as:
#define THREAD_LOCAL(_x) __thread _x
...
static THREAD_LOCAL(xo_handle_t) xo_default_handle;
To help debug this issue, I made the following change to the sources
to help with gdb's inability to show thread-local variables ("Cannot
find thread-local variables on this target"):
--- contrib/libxo/libxo/libxo.c.save 2018-05-04 17:26:29.079500000 =
-0400
+++ contrib/libxo/libxo/libxo.c 2018-05-04 17:28:06.570875000 -0400
@@ -8349,3 +8349,11 @@
xop->xo_style =3D XO_STYLE_ENCODER;
xop->xo_encoder =3D encoder;
}
+
+void xo_print_handle (void);
+void
+xo_print_handle (void)
+{
+ fprintf(stderr, "xo_default_handle: %p %d\n",
+ &xo_default_handle, sizeof(xo_handle_t));
+}
When I run the failing command (uptime) under gdb and breakpoint
on main, my thread-local variable is not set to zeroes:
% gdb uptime
GNU gdb 6.1.1 [FreeBSD]
...
This GDB was configured as "i386-marcel-freebsd"...
(gdb) b main
Breakpoint 1 at 0x8049be5: file /usr/src/usr.bin/w/w.c, line 145.
(gdb) run
Starting program: /usr/home/phil/work/lib/uptime
Breakpoint 1, main (argc=3D1, argv=3D0xbfbfe60c) at /usr/src/usr.bin/w=
/w.c:145
145 (void)setlocale(LC_ALL, "");
Current language: auto; currently minimal
(gdb) call xo_print_handle()
xo_default_handle: 0x2806aff0 328
$1 =3D 34
(gdb) x/82x 0x2806aff0
0x2806aff0: 0x00000000 0x00000000 0x00000000 0x2806=
01ef
0x2806b000: 0x2806b010 0x2806a200 0x00000001 0x2806=
01ef
0x2806b010: 0x2806b020 0x2806a400 0x0000005d 0x2806=
01ef
0x2806b020: 0x2806b030 0x2806a600 0x000000a1 0x2806=
01ef
0x2806b030: 0x2806b040 0x2806a800 0x00000147 0x2806=
01ef
0x2806b040: 0x00000000 0x2806aa00 0x00000164 0x2806=
01ef
0x2806b050: 0x2806c000 0x00000000 0x28065e70 0x2806=
01ef
0x2806b060: 0x2806b070 0x2806ac00 0x00000421 0x2806=
01ef
0x2806b070: 0x00000000 0x2806aa00 0x0000042d 0x2806=
01ef
0x2806b080: 0x00000000 0x2806aa00 0x000001ff 0x2806=
01ef
0x2806b090: 0x2806b0a0 0x2806a800 0x00000976 0x2806=
01ef
0x2806b0a0: 0x00000000 0x2806aa00 0x00000983 0x2806=
01ef
0x2806b0b0: 0x00000000 0x2806aa00 0x00000a18 0x2806=
01ef
0x2806b0c0: 0x00000000 0x2806aa00 0x00000571 0x2806=
01ef
0x2806b0d0: 0x2806b0e0 0x2806a000 0x00000000 0x2806=
01ef
0x2806b0e0: 0x2806b0f0 0x2806a200 0x00000000 0x2806=
01ef
0x2806b0f0: 0x2806b100 0x2806a400 0x00000000 0x2806=
01ef
0x2806b100: 0x2806b110 0x2806a600 0x00000000 0x2806=
01ef
0x2806b110: 0x2806b120 0x2806a800 0x00000000 0x2806=
01ef
0x2806b120: 0x2806b130 0x2806aa00 0x00000000 0x2806=
01ef
0x2806b130: 0x00000000 0x2806ac00
(gdb)
objdump shows the lib does have a .tbbs:
14 .tbss 00000658 000181f8 000181f8 000171f8 2**3
ALLOC, THREAD_LOCAL
Thanks,
Phil
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