[Bug libc/19356] New: unwind stack failed if stack including syscall on armv7

["guonic at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=19356

            Bug ID: 19356
           Summary: unwind stack failed if stack including syscall on
                    armv7
           Product: glibc
           Version: 2.20
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: guonic at gmail dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Scenario:
using libunwind in a signal handler to output backtrace, always failed if
backtrace including syscall function.
Wrong Backtrace:
   1 ip:0xb6443990 sp:0xbe9c38a8 /usr/lib/libc-2.20.so(syscall + 32)            
   2 ip:0xb6e9d0af sp:0xbe9c38a8 /usr/lib/libuv.so.11.0.0(uv__epoll_wait + 22)  
   3 ip:0xb6e9d2f8 sp:0xbe9c38c0 /usr/lib/libuv.so.11.0.0(__aeabi_uldivmod +
44)  

Right Backtrace:
   1 ip:0xb63d7990 sp:0xbeab48a8 /usr/lib/libc-2.20.so(syscall + 32) 
   2 ip:0xb6e310af sp:0xbeab48b8 /usr/lib/libuv.so.11.0.0(uv__epoll_wait + 22) 
   3 ip:0xb6e300a7 sp:0xbeab48d0 /usr/lib/libuv.so.11.0.0(uv__io_poll + 266)
   4 ip:0xb6e285a1 sp:0xbeab8918 /usr/lib/libuv.so.11.0.0(uv_run + 144)
   5 ip:0xb66af495 sp:0xbeab8950 /usr/lib/libbase.so.1.0.0(yunos::Looper::run()
+ 72) 
   6 ip:0xb6e8dd9b sp:0xbeab8960 /usr/lib/libnode.so(node::Start(int, char**) +
1050)
   7 ip:0xb631d6d4 sp:0xbeab8ae8 /usr/lib/libc-2.20.so(__libc_start_main + 276)
   8 ip:0x10800 sp:0xbeab8c28 /usr/bin/node(_start + 44) 

After investigate with syscall & unwind info with readelf/objdump, the root
cause is wrong unwind info provided from libc-2.20.so


Dissembled code of syscall:

000d1948 <setlogmask>:                                                          
   d1948:       e59f3010        ldr     r3, [pc, #16]   ; d1960
<setlogmask+0x18>                                     
   d194c:       e2502000        subs    r2, r0, #0                              
   d1950:       e08f3003        add     r3, pc, r3                              
   d1954:       e593000c        ldr     r0, [r3, #12]                           
   d1958:       1583200c        strne   r2, [r3, #12]                           
   d195c:       e12fff1e        bx      lr                                      
   d1960:       00078950        andeq   r8, r7, r0, asr r9                      
        ...                                                                     

000d1970 <syscall>:                                                             
   d1970:       e1a0c00d        mov     ip, sp                                  
   d1974:       e92d00f0        push    {r4, r5, r6, r7}                        
   d1978:       e1a07000        mov     r7, r0                                  
   d197c:       e1a00001        mov     r0, r1                                  
   d1980:       e1a01002        mov     r1, r2                                  
   d1984:       e1a02003        mov     r2, r3                                  
   d1988:       e89c0078        ldm     ip, {r3, r4, r5, r6}                    
   d198c:       ef000000        svc     0x00000000                              
   d1990:       e8bd00f0        pop     {r4, r5, r6, r7}     ######## PC point
here                                                             
   d1994:       e3700a01        cmn     r0, #4096       ; 0x1000                
   d1998:       312fff1e        bxcc    lr                                      
   d199c:       eafd17a3        b       17830 <__libc_start_main+0x270>    



Unwind info of syscall:
0xd1890: @0x12fe40
  Personality routine: 0x17a80

0xd1948: 0x80b0b0b0         #####  Executed unwind frame if PC was in syscall 
  Compact model index: 0
  0xb0      finish
  0xb0      finish
  0xb0      finish

0xd19a0: 0x801aabb0
  Compact model index: 0
  0x1a      vsp = vsp + 108
  0xab      pop {r4, r5, r6, r7, r14}
  0xb0      finish


Actually no syscall unwind info in .arm.exidx section, the executed unwind info
is pointed 0xd1948, it failed after frame point to uv__epoll_wait, because of
lost to 
pop four register (R4, R5, R6, R7} on syscall frame.

I have write the patch to execute unwind info when pc point to syscalll(addr:
d1990) ike this:
  0xa3      pop {r4, r5, r6, r7}
  0xb0      finish
  0xb0      finish
then get the right backtrace like gdb output.

Is there anything i can do to get the right unwind info from libc-2.20.so
without to change libunwind? 

Appreciated in advance!!!!

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