Bug 995446 - gcc -02 optmization regression in v4.8 causing malformed arg passing (32 bit arch only)
gcc -02 optmization regression in v4.8 causing malformed arg passing (32 bit ...
Product: Fedora
Classification: Fedora
Component: gcc (Show other bugs)
i386 Linux
high Severity urgent
: ---
: ---
Assigned To: Jakub Jelinek
Fedora Extras Quality Assurance
Depends On:
Blocks: 996643
  Show dependency treegraph
Reported: 2013-08-09 07:42 EDT by Peter Rajnoha
Modified: 2013-09-30 06:13 EDT (History)
9 users (show)

See Also:
Fixed In Version:
Doc Type: Bug Fix
Doc Text:
Story Points: ---
Clone Of:
: 996643 (view as bug list)
Last Closed: 2013-09-30 06:13:54 EDT
Type: Bug
Regression: ---
Mount Type: ---
Documentation: ---
Verified Versions:
Category: ---
oVirt Team: ---
RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: ---

Attachments (Terms of Use)
preprocessed lib/config/config.c from lvm2 source tree (267.38 KB, text/plain)
2013-08-12 07:09 EDT, Peter Rajnoha
no flags Details
rh995446.i (2.44 KB, text/plain)
2013-08-13 05:31 EDT, Jakub Jelinek
no flags Details

External Trackers
Tracker ID Priority Status Summary Last Updated
GNU Compiler Collection 57459 None None None Never

  None (edit)
Description Peter Rajnoha 2013-08-09 07:42:15 EDT
We've just hit this. It seems that there's some regression in -O2 gcc optimization which causes the value to be malformed on function call (the value is incresed by one). We've hit this problem with lvm2 with recent gcc in F19 (gcc (GCC) 4.8.1 20130603 (Red Hat 4.8.1-1)).

Snippet from the log message for the workaround we use now in lvm2 to avoid this issue:

 gcc -O2 v4.8 on 32 bit architecture is causing a bug in parameter
    passing. It does not happen with -01 nor -O0.
    The problematic part of the code was strlen use in config.c in
    the config_def_check fn and the call for _config_def_check_tree in it:
      rplen = strlen(rp);
      if (!_config_def_check_tree(handle, vp, vp + strlen(vp), rp, rp + rplen, CFG_PATH_MAX_LEN - rplen, cn, cmd->cft_def_hash)) ...

   If compiled with -O0 (correct):
    Breakpoint 1, config_def_check (cmd=0x819b050, handle=0x81a04f8) at config/config.c:775
    (gdb) p	vp
    $1 = 0x8189ee0 <_cfg_path> "config"
    (gdb) p	strlen(vp)
    $2 = 6
    _config_def_check_tree (handle=0x81a04f8, vp=0x8189ee0 <_cfg_path>
    "config", pvp=0x8189ee6 <_cfg_path+6> "", rp=0xbfffe1e8 "config",
    prp=0xbfffe1ee "", buf_size=58, root=0x81a2568, ht=0x81a65
    48) at config/config.c:680
    (gdb) p	vp
    $4 = 0x8189ee0 <_cfg_path> "config"
    (gdb) p	pvp
    $5 = 0x8189ee6 <_cfg_path+6> ""
    If compiled with -O2 (incorrect):
    Breakpoint 1, config_def_check (cmd=cmd@entry=0x8183050, handle=0x81884f8) at config/config.c:775
    (gdb) p	vp
    $1 = 0x8172fc0 <_cfg_path> "config"
    (gdb) p strlen(vp)
    $2 = 6
    (gdb) p	vp + strlen(vp)
    $3 = 0x8172fc6 <_cfg_path+6> ""
    _config_def_check_tree (handle=handle@entry=0x81884f8, pvp=0x8172fc7
    <_cfg_path+7> "host_list", rp=rp@entry=0xbffff190 "config",
    prp=prp@entry=0xbffff196 "", buf_size=buf_size@entry=58, ht=0x
    818e548, root=0x818a568, vp=0x8172fc0 <_cfg_path> "config") at
    (gdb) p	pvp
    $4 = 0x8172fc7 <_cfg_path+7> "host_list"
    The difference is in passing the "pvp" arg for _config_def_check_tree.
    While in the correct case, the value of _cfg_path+6 is passed
    (the result of vp + strlen(vp) - see the snippet of the code above),
    in the incorrect case, this value is increased by 1 to _cfg_path+7,
    hence totally malforming the string that is being processed.

See also https://git.fedorahosted.org/cgit/lvm2.git/commit/?id=2f61478436a7ebe058ffad3f92e1c4dada0805d0. The workaround we provide for now is that the _cfg_path variable is not define with "const" qualifier (so we assume that the optimizer is less aggressive then - it actually helps).

The code itself: https://git.fedorahosted.org/cgit/lvm2.git/tree/lib/config/config.c. The breakpoint was set to the line just before the _config_def_check_tree_call in config_def_check function.

Please, let me know if you need any more info...
Comment 1 Peter Rajnoha 2013-08-09 07:45:32 EDT
(In reply to Peter Rajnoha from comment #0)

> ?id=2f61478436a7ebe058ffad3f92e1c4dada0805d0. The workaround we provide for
> now is that the _cfg_path variable is not define with "const" qualifier (so
> we assume that the optimizer is less aggressive then - it actually helps).

(sorry, I meant "static" qualifier, not the "const").

Also, shuffling the arg list for the _config_def_check_tree fn helped (but the workaround with removing the "static" is shorter...
Comment 2 Peter Rajnoha 2013-08-09 08:04:47 EDT
This is not reproducible with the older gcc v4.7.2 (the version that is currently in F18).
Comment 3 Jakub Jelinek 2013-08-12 05:35:56 EDT
Please provide preprocessed source for the source file in question and gcc command line options used to compile it.
Comment 4 Peter Rajnoha 2013-08-12 07:09:51 EDT
Created attachment 785648 [details]
preprocessed lib/config/config.c from lvm2 source tree

Preprocessed config.i attached, the complete option list used:

-Wall -Wundef -Wshadow -Wcast-align -Wwrite-strings -Wmissing-prototypes -Wmissing-declarations -Wnested-externs -Winline -Wmissing-noreturn -Wformat-security -Wredundant-decls -Wpointer-arith -g -O2 -fPIC -save-temps
Comment 5 Jakub Jelinek 2013-08-12 15:37:13 EDT
I think I'd prefer to see this under debugger, how can one reproduce this under gdb and does it require any special setup?
Comment 6 Mikuláš Patočka 2013-08-12 21:20:43 EDT
The expanded strlen of an aligned string should look like this:
        movl    $b, %eax
        movl    (%eax), %ecx
        addl    $4, %eax
        leal    -16843009(%ecx), %edx
        notl    %ecx
        andl    %ecx, %edx
        andl    $-2139062144, %edx
        je      .L2
        movl    %edx, %ecx
        shrl    $16, %ecx
        testl   $32896, %edx
        cmove   %ecx, %edx
        leal    2(%eax), %ecx
        cmove   %ecx, %eax
        addb    %dl, %dl
        sbbl    $3, %eax
        subl    $b, %eax

In the assembler that results from compiling the submitted file with
"gcc -m32 -O2 -g -fPIC config.i -S" we can see:

        movl    (%edx), %ecx
        addl    $4, %edx
        leal    -16843009(%ecx), %eax
        notl    %ecx
        movl    %eax, 44(%esp)
        andl    %ecx, 44(%esp)
        andl    $-2139062144, 44(%esp)
        je      .L706
        movl    44(%esp), %eax
        movl    52(%esp), %ecx
        movl    %ecx, 60(%esp)
        movl    %eax, %ecx
        shrl    $16, %ecx
        testl   $32896, %eax
        movl    %ecx, 52(%esp)
        movl    %eax, %ecx
        leal    2(%edx), %eax
        cmove   52(%esp), %ecx
        cmove   %eax, %edx
        addb    %cl, 44(%esp)
        .loc 1 775 0
        movl    160(%esp), %eax
        .loc 1 776 0
        sbbl    $3, %edx
        subl    48(%esp), %edx
        .loc 1 775 0
        addl    56(%esp), %edx
        movl    %edx, %ecx

--- that "addb    %cl, 44(%esp)" seems to be wrong, it should be "addb %cl, %cl"
Comment 7 Jakub Jelinek 2013-08-13 05:31:10 EDT
Created attachment 786091 [details]

Self-contained reduced executable testcase.
Comment 8 Jakub Jelinek 2013-08-13 05:41:02 EDT
Seems to be PR57459.
Comment 9 Jakub Jelinek 2013-09-30 06:13:54 EDT
Fixed in 4.8.1-6 and later.

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