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Re: [Libunwind-devel] unw_init_remote call results in SIGSEGV

From: Lassi Tuura
Subject: Re: [Libunwind-devel] unw_init_remote call results in SIGSEGV
Date: Mon, 20 Apr 2015 20:39:54 +0200
It doesn't look like you are initialising your unw_accessors_t accessors. From libunwind-ptrace man page:

An application that wants to use the _UPT-facility first needs to create a new libunwind address-space that represents the target process. This is done by calling unw_create_addr_space(). In many cases, the application will simply want to pass the address of _UPT_accessors as the first argument to this routine

On Mon, Apr 20, 2015 at 1:42 PM, Arun Prakash Jana <address@hidden> wrote:
Hi,

I am writing a small test program to check how libunwind can be used to trace a remote program.

A call to unw_init_remote() throws SIGSEGV.

Relevant code snippet:
------------------
        unw_addr_space_t addrspace;
        struct UPT_info *uptinfo;
        unw_accessors_t accessors;
        unw_cursor_t cursor;
        unw_word_t RIP, RBP;

        pid_t PID = 1;
        int ret = 0;

        if (argc !=2) {
                fprintf(stderr, "Usage: unwind PID\n");
                return -1;
        }

        if ((PID = atoi(argv[1])) <= 0) {
                fprintf(stderr, "Valid PID please!\n");
                return -1;
        }

        fprintf(stdout, "Tracing PID: %d\n", PID);

        /* Create address space for little endian */
        addrspace = unw_create_addr_space(&accessors, 0);
        if (!addrspace) {
                fprintf(stderr, "unw_create_addr_space failed\n");
                return -1;
        }

        uptinfo = (struct UPT_info *)_UPT_create(PID);
        if (!uptinfo) {
                fprintf(stderr, "_UPT_create failed\n");
                goto bail;
        }

        ret = unw_init_remote(&cursor, addrspace, uptinfo);
        if (ret < 0) {
                fprintf(stderr, "unw_init_remote failed\n");
                goto bail;
        }
------------------

strace output:
------------------
> strace ./unwind 8423
execve("./unwind", ["./unwind", "8423"], [/* 85 vars */]) = 0
brk(0)                                  = 0x1cdd000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc136cc9000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=81750, ...}) = 0
mmap(NULL, 81750, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fc136cb5000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/usr/lib/x86_64-linux-gnu/libunwind-x86_64.so.8", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0p\27\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=67608, ...}) = 0
mmap(NULL, 2223816, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fc13688a000
mprotect(0x7fc13689a000, 2093056, PROT_NONE) = 0
mmap(0x7fc136a99000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0xf000) = 0x7fc136a99000
mmap(0x7fc136a9b000, 57032, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fc136a9b000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/usr/lib/x86_64-linux-gnu/libunwind-ptrace.so.0", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\17\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=14528, ...}) = 0
mmap(NULL, 2109736, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fc136686000
mprotect(0x7fc136689000, 2093056, PROT_NONE) = 0
mmap(0x7fc136888000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x2000) = 0x7fc136888000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\320\37\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1840928, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc136cb4000
mmap(NULL, 3949248, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fc1362c1000
mprotect(0x7fc13647c000, 2093056, PROT_NONE) = 0
mmap(0x7fc13667b000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1ba000) = 0x7fc13667b000
mmap(0x7fc136681000, 17088, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fc136681000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/usr/lib/x86_64-linux-gnu/libunwind.so.8", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0000\37\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=51192, ...}) = 0
mmap(NULL, 2207464, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fc1360a6000
mprotect(0x7fc1360b2000, 2093056, PROT_NONE) = 0
mmap(0x7fc1362b1000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0xb000) = 0x7fc1362b1000
mmap(0x7fc1362b3000, 57064, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fc1362b3000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/liblzma.so.5", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\320 \0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=137400, ...}) = 0
mmap(NULL, 2232456, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fc135e84000
mprotect(0x7fc135ea5000, 2093056, PROT_NONE) = 0
mmap(0x7fc1360a4000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x20000) = 0x7fc1360a4000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\320\16\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=14664, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc136cb3000
mmap(NULL, 2109744, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fc135c80000
mprotect(0x7fc135c83000, 2093056, PROT_NONE) = 0
mmap(0x7fc135e82000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x2000) = 0x7fc135e82000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc136cb2000
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc136cb0000
arch_prctl(ARCH_SET_FS, 0x7fc136cb0780) = 0
mprotect(0x7fc13667b000, 16384, PROT_READ) = 0
mprotect(0x7fc135e82000, 4096, PROT_READ) = 0
mprotect(0x7fc1360a4000, 4096, PROT_READ) = 0
mprotect(0x7fc1362b1000, 4096, PROT_READ) = 0
mprotect(0x7fc136888000, 4096, PROT_READ) = 0
mprotect(0x7fc136a99000, 4096, PROT_READ) = 0
mprotect(0x601000, 4096, PROT_READ)     = 0
mprotect(0x7fc136ccb000, 4096, PROT_READ) = 0
munmap(0x7fc136cb5000, 81750)           = 0
fstat(1, {st_mode=S_IFREG|0664, st_size=5320, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc136cc8000
brk(0)                                  = 0x1cdd000
brk(0x1d08000)                          = 0x1d08000
rt_sigprocmask(SIG_SETMASK, ~[RTMIN RT_1], [], 8) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc136cc7000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc136cc6000
mincore(0x7ffd4ae65b3f, 1, 0x7ffd4ae65b3f) = -1 EINVAL (Invalid argument)
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
--- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=0} ---
+++ killed by SIGSEGV +++
fish: Job 1, “strace ./unwind 8423 > strace.log 2>&1” terminated by signal SIGSEGV (Address boundary error)
------------------

I am on Ubuntu 14.04 (arch: x86_64). Can someone please explain what I'm doing wrong?

Regards,

Arun

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