Prelink prelink elf shared libraries and binaries
The reason prelink does this is because kernel facilities supplying address space layout randomization ASLR for libraries cannot be used in conjunction with prelink without defeating the purpose of prelink and forcing the dynamic linker to perform relocations at program load time. As stated, prelink and per-process library address randomization cannot be used in conjunction.
In order to avoid completely removing this security enhancement, prelink supplies its own randomization; however, this does not help a general information leak caused by prelink. Attackers with the ability to read certain arbitrary files on the target system can discover where libraries are loaded in privileged daemons; often libc is enough as it is the most common library used in return-to-libc attacks.
By reading a shared library file such as libc, an attacker with local access can discover the load address of libc in every other application on the system.
Since most programs link to libc, the libc library file always has to be readable; any attacker with local access may gather information about the address space of higher privileged processes. Local access may commonly be gained by shell accounts or Web server accounts that allow the use of CGI scripts, which may read and output any file on the system.
Because prelink is often run periodically, typically every two weeks, the address of any given library has a chance of changing over time. This gives any address derived a half-life of the period in which prelink is run. Also note that if a new version of the library is installed, the addresses changes. Occasionally prelinking can cause issues with application checkpoint and restart libraries like blcr,  as well as other libraries like OpenMPI that use blcr internally.
Specifically when checkpointing a program on one host, and trying to restart on a different host, the restarted program may fail with a segfault due to differences in host-specific library memory address randomization.
From Wikipedia, the free encyclopedia. This article has multiple issues. Also note that if a new version of the library is installed, the addresses changes. Occasionally prelinking can cause issues with application checkpoint and restart libraries like blcr,  as well as other libraries like OpenMPI that use blcr internally. Specifically when checkpointing a program on one host, and trying to restart on a different host, the restarted program may fail with a segfault due to differences in host-specific library memory address randomization.
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As mentioned earlier, the ELF's 'execution view' is concerned with how to load an executable binary into memory. Also note that each of libc The two [anon] memory segments at 0x and 0x are for sections which do not take space in the ELF binary files. For example, readelf -t xxx. The above debugging information does not show mmap and mprotect calls. However, we can use strace. If we run the user program again with.
For the compiler part, GCC uses different prolog and epilog files, depending on the compiler command-line options. To see them, execute gcc -dumpspec , and one can see. Similarly, if -shared compiler command-line option is not used, then always include Glibc's crt1. Next, include Glibc's crti. Finally, include either crtbeginT. So, for example, if a program is compiled using dynamic linking which is default , no profiling, no fast math optimizations, then the linking will include the following files in the following order:.
It initializes gprof related data structures. It is not advisable to put a code in. Similarly, it is not advisable to put a code in. To see this, run gcc with -v command, and the last line would be something like:. Of course, if the user program calls exit or abort , then exit will gets called. If one tries to build a program which does not contain main , then one should see the following error:.
From above analysis, it's possible to find out the address of main which is NOT the "Entry point address" seen from the output of readelf -h a. On bit x86 , the calling convention requires that the first argument goes to RDI register , so the address can be extracted by. According to Chapter 3. The readelf -d a. What does this prelink do?
It changes the base address of a dynamic library to the actual address in the user program's address space when it is loaded into memory. Normally, a dynamic library is built as position independent code , i. For example, a normal libc. How to disable prelinking at runtime? First to be processed is the. If prelink is used, i. The next to be processed by ld. This time, the address returned is the runtime address of foo in libfoo.
As mentioned earlier, this address holds the initial value of foo. The above example also illustrates the difference between. For the runtime linker ld. Since the relocation of both bar and printf are in. So how does ld. So it has to be relocated and patched as b0. This usually happens when the dynamic binary in question is built using newer version of GCC.