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Date: Mon, 13 Nov 2017 15:09:33 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: diffutils crash in malloc

On Mon, Nov 13, 2017 at 08:43:50PM +0100, Tobias Koch wrote:
> Hi,
> 
> I run two stacks in parallel: the target stack emulated with Qemu
> and a native stack in the /tools folder. Both are cross-compiled
> from a glibc-based system. In very simple cases, an application in
> the tools folder is configured via
> 
> ../configure --host=x86_64-cross-linux-musl --prefix=/tools
> 
> The diffutils build system seems to insist on putting libc.so in the
> needed library section twice. Only the RUNPATH entry I could get rid
> of via --disable-rpath. An strace of the crashing diff invocation
> looks like this:
> 
> execve("/tools/bin/diff", ["/tools/bin/diff", "a", "b"], [/* 33 vars
> */]) = 0
> arch_prctl(ARCH_SET_FS, 0x7f5011a98b68) = 0
> set_tid_address(0x7f5011a98ba0)         = 26946
> open("/tools/lib/libc.so", O_RDONLY|O_CLOEXEC) = 3
> fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
> fstat(3, {st_mode=S_IFREG|0755, st_size=3816240, ...}) = 0
> read(3,
> "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\00055\7\0\0\0\0\0"...,
> 960) = 960
> mmap(NULL, 2772992, PROT_READ|PROT_EXEC, MAP_PRIVATE, 3, 0) = 0x7f5011550000
> mmap(0x7f50117f0000, 20480, PROT_READ|PROT_WRITE,
> MAP_PRIVATE|MAP_FIXED, 3, 0xa0000) = 0x7f50117f0000
> mmap(0x7f50117f2000, 12288, PROT_READ|PROT_WRITE,
> MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f50117f2000
> close(3)                                = 0
> munmap(0x7f5011550000, 2772992)         = 0

OK, you've found a bug in the new code for avoiding multiple libc
instances getting loaded if it happens at program-load time rather
than via dlopen at runtime. The code that reclaims slack space at the
edges of the writable LOAD mapping for the newly loaded library runs
before checking if the library is a duplicate of libc. If it's found
to be a duplicate, it's unloaded, and the space that was just donated
to malloc is invalid.

The attached patch should correct the issue. Note that, in order for
it to help, the copy of musl that's in /lib/ld-musl-x86_64.so.1 needs
to be updated; it is the only one that is actually running/used. If
you really want the one in /tools to be used, your $CC or $LDFLAGS
needs to include -Wl,-dynamic-linker,/tools/lib/ld-musl-x86_64.so.1 or
similar.

Rich

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