IA64: Incomplete unwind description for strncpy, strcat, _mcount

[Patil, Harish]

>Submitter-Id:  net
>Originator:    
>Organization: Intel
>Confidential:  no
>Synopsis:  Incomplete unwind description for strncpy, strcat, _mcount
>Severity:  non-critical
>Priority:  medium
>Category:  libc
>Class:     sw-bug 
>Release:   libc-2.2.4
>Environment:
Host type: ia64-unknown-linux-gnu
System: Linux shli0002 2.4.18 #1 SMP Fri May 31 13:56:22 EDT 2002 ia64
unknown
Architecture: ia64

Addons: linuxthreads
Build CFLAGS: -O2 -g
Build CC: gcc32
Compiler version: 3.2
Kernel headers: 2.4.9-18
Symbol versioning: yes
Build static: yes
Build shared: yes
Build pic-default: no
Build profile: yes
Build omitfp: no
Build bounded: no
Build static-nss: no
Stdio: libio

>Description:
    The test program uses strncpy and strcat.

    Looking at the output of "readelf -u" on the test program the unwind
     info for strncpy says:

    R2:prologue_gr(mask=[rp,ar.pfs],grsave=r35,rlen=5)

    This is a shorthand for telling that rp is saved in r35 and ar.pfs
    is saved in r36. This is incomplete because it tells 'where' rp and pfs
    are saved but does not tell 'when' they are saved i.e. a P7:pfs_when and
    P7:rp_when are missing in the unwind info.

    Function strcat has the same problem as shown in the test case. Looking
    at glibc source function _mcount has the same problem.

>How-To-Repeat:
BEGIN strtest.c
    #include <stdio.h>
    void  do_strncpy()
    {
    char a[4] = "hia";
    char b[4] = "hib";
    strncpy(a,b,3);
    }
    void  do_strcat()
    {
    char a[4] = "hia";
    char b[5] = "";
    strcat(a,b);
    }

    main()
    {
    do_strncpy();
    do_strcat();
    printf("Done\n");
    }
END strtest.c
    gcc -static strtest.c -o strtest
    readelf -u strtest > strtest.unw
    vi strtest.unw

>Fix:
    Looking at: sysdeps/ia64/strncpy.S
        ENTRY(strncpy)
        .prologue ASM_UNW_PRLG_RP|ASM_UNW_PRLG_PFS, ASM_UNW_PRLG_GRSAVE(3)
        alloc saved_pfs = ar.pfs, 3, 6, 3, 0
        mov saved_b0 = b0

    We see that pfs is saved in t==0 and rp is saved in t==1.

    Problem: the assembly directive ".prologue mask, reg" has no way to
        specify the 'time' where save actually occurs.
    Possible fix: Use the more verbose ".save" directive so the assembly
    would look like:
        .prologue
        .save pfs, saved_pfs
        alloc saved_pfs = ar.pfs, 3, 6, 3, 0
        .save rp, saved_b0
        mov saved_b0 = b0

    The result will be a slightly expanded unwind info: so instead of the
    compact R2:prologue_gr(mask=[rp,ar.pfs],grsave=r35,rlen=5) the assembler
    will generate two records P3:rp_gr(reg=35) and P3:pfs_gr(reg=36) but it
    will also correctly generate the currently missing P7:pfs_when(t=0) and
    P7:rp_when(t=1).

    strcat.S and _mcount.S also use
        .prologue ASM_UNW_PRLG_RP|ASM_UNW_PRLG_PFS, ASM_UNW_PRLG_GRSAVE(3)
    hence have the same problem/fix.