Re: Infinite loop in malloc

[Szabolcs Nagy <nsz@port70.net>]

* r yang <decatf@gmail.com> [2019-01-25 10:13:50 -0500]:
> pmbootstrap is a development environment to build/install postmarketOS
> (based on Alpine Linux) for Android devices. One of the things it does
> is use qemu static to emulate an ARM based Alpine Linux chroot
> environment.
> 
> There is a bug while compiling certain packages in the qemu ARM chroot.
> The qemu process can get stuck in an infinite loop when calling malloc.
> 
> pmbootstrap uses Alpine Linux edge repositories. It's using the current
> musl package version 1.1.20.
> 
> Here is a gdb backtrace.
> #0  malloc (n=<optimized out>, n@entry=9) at src/malloc/malloc.c:320
> #1  0x0000000060184ad3 in g_malloc (n_bytes=n_bytes@entry=9) at gmem.c:99
> #2  0x000000006018bcab in g_strdup (str=<optimized out>, str@entry=0x60200abf "call_rcu") at gstrfuncs.c:363
> #3  0x000000006016e31d in qemu_thread_create (thread=thread@entry=0x7ffe89fb1a10, name=name@entry=0x60200abf "call_rcu",
>     start_routine=start_routine@entry=0x60174c00 <call_rcu_thread>, arg=arg@entry=0x0, mode=mode@entry=1) at /home/pmos/build/src/qemu-3.1.0/util/qemu-thread-posix.c:526
> #4  0x0000000060174b99 in rcu_init_complete () at /home/pmos/build/src/qemu-3.1.0/util/rcu.c:327
> #5  0x00000000601c4fac in __fork_handler (who=1) at src/thread/pthread_atfork.c:26
> #6  0x00000000601be8db in fork () at src/process/fork.c:33
> #7  0x000000006009d191 in do_fork (env=0x62ef0ed0, flags=flags@entry=17, newsp=newsp@entry=0, parent_tidptr=parent_tidptr@entry=0, newtls=newtls@entry=0,
>     child_tidptr=child_tidptr@entry=0) at /home/pmos/build/src/qemu-3.1.0/linux-user/syscall.c:5528
> #8  0x00000000600af894 in do_syscall1 (cpu_env=cpu_env@entry=0x62ef0ed0, num=num@entry=2, arg1=arg1@entry=0, arg2=arg2@entry=-8700192, arg3=<optimized out>, arg4=8,
>     arg5=1015744, arg6=-75664, arg7=0, arg8=0) at /home/pmos/build/src/qemu-3.1.0/linux-user/syscall.c:7042
> #9  0x00000000600a835c in do_syscall (cpu_env=cpu_env@entry=0x62ef0ed0, num=2, arg1=0, arg2=-8700192, arg3=<optimized out>, arg4=<optimized out>, arg5=1015744, arg6=-75664,
>     arg7=0, arg8=0) at /home/pmos/build/src/qemu-3.1.0/linux-user/syscall.c:11533
> #10 0x00000000600c265f in cpu_loop (env=env@entry=0x62ef0ed0) at /home/pmos/build/src/qemu-3.1.0/linux-user/arm/cpu_loop.c:360
> #11 0x00000000600417a2 in main (argc=<optimized out>, argv=0x7ffe89fb5958, envp=<optimized out>) at /home/pmos/build/src/qemu-3.1.0/linux-user/main.c:819
> 
> 
> It is taking the malloc code path where n <= MMAP_THRESHOLD. None of
> the conditions which break from the for loop are met.
> 
> In the first condition the mask value is never zero:
>     mask = mal.binmap & -(1ULL<<i);
>     if (!mask) { ... }
> 
> Examining the value in gdb:
> (gdb) printf "%X\n", mask
> 204701
> 
> The bin head points to the bin itself so this condition is never met:
>     c = mal.bins[j].head;
>     if (c != BIN_TO_CHUNK(j)) { ... }
> 
> Examining the values in gdb:
> (gdb) printf "%X\n", mal.bins[j].head
> 62337FC0
> (gdb) printf "%X\n", (struct chunk *)((char *)(&mal.bins[j].head) - (2*sizeof(size_t)))
> 62337FC0
> 
> 
> Reproducing this issue:
> It is not always 100% reproducible. On occasion it will not get stuck
> in an infinite loop. With my testing on 2 computers, will happen on
> most attempts to compile.

thanks i managed to reproduce this on my laptop with the commands below.
i'll try to look into it.

> 
> $ git clone https://gitlab.com/postmarketOS/pmbootstrap.git
> $ cd pmboostrap
> 
> Configure pmbootstrap
> $ ./pmbootstrap.py init
> 
> Enter an Android device when prompted.
> Use device: samsung-i9100
> Leave other settings as the default.
> 
> Check out the pmaports repository that will reproduce this issue.
> $ cd /path/to/pmboostrap/aports
> $ git remote add ryang2678 https://gitlab.com/ryang2678/pmaports.git
> $ git fetch ryang2678 debug-musl-malloc
> $ git checkout debug-musl-malloc
> 
> Compile qemu static with debug symbols.
> Alpine Linux doesn't provide a qemu package with debug symbols.
> The debug-musl-malloc branch contains a qemu APKBUILD with debugging
> enabled.
> $ cd /path/to/pmboostrap
> $ ./pmbootstrap.py build qemu
> 
> Try to compile networkmanager and wait for build to get stuck.
> $ ./pmbootstrap.py build networkmanager --arch=armhf --force
> 
> 
> To observe the stuck qemu process:
> 
> Enter chroot shell:
> $ ./pmbootstrap.py chroot
> 
> Install musl debug symbols.
> $ apk add musl-dbg
> 
> Get musl source code
> $ cd /home/pmos
> $ git clone git://git.musl-libc.org/musl
> $ cd /home/pmos/musl
> $ git checkout v1.1.20
> 
> Attach gdb to stuck process
> $ gdb -tui /usr/bin/qemu-arm
> directory /home/pmos/musl
> attach <pid>