The most difficult part of setting up a chroot environment is identifying dependencies for the programs you want to copy to the jail. For example, to make cp available, not only do you need to copy its binary from /bin and any shared libraries it depends on, but the dependencies can have their own dependencies too that need to be copied. The internet suggests using ldd to list a binary’s dependencies, but that has its own problems. The man page for ldd warns not to use the script for untrusted programs because it works by setting a special environment variable and then executes the program. What’s a security-conscious systems administrator to do? The ldd man page recommends objdump as a safe alternative. objdump outputs information about an object file, including what shared libraries it links against. It doesn’t identify the dependencies’ dependencies, but it’s still a good start because it doesn’t try to execute the target file. We can overcome the dependencies of depende...
In preparation for a presentation I’m giving at this month’s Syracuse PHP Users Group meeting, I found the need to read in Unicode characters in PHP one at a time. Unicode is still second-class in PHP; PHP6 failed and we have to fallback to extensions like the mbstring extension and/or libraries like Portable UTF-8 . And even with those, I didn’t see a unicode-capable fgetc() so I wrote my own. Years ago, I wrote a post describing how to read Unicode characters in C , so the logic was already familiar. As a refresher, UTF-8 is a multi-byte encoding scheme capable of representing over 2 million characters using 4 bytes or less. The first 128 characters are encoded the same as 7-bit ASCII with 0 as the most-significant bit. The other characters are encoded using multiple bytes, each byte with 1 as the most-significant bit. The bit pattern in the first byte of a multi-byte sequence tells us how many bytes are needed to represent the character. Here’s what the function looks like: f...