Archive for December, 2010

Concatenate PDF files in Linux

By chys on December 26th, 2010

Some people recommend using convert or gs. However, there is a major problem with them – all text and vector graphics become raster graphics.

pdftk (PDF ToolKit) is a better solution – it keeps text and vector graphics. We just have to use this command:

pdftk *.pdf cat output result.pdf

PDF Shuffler, written in Python and based on poppler, also does the trick, and has a nice GUI.

However, there are drawbacks for both pdftk and PDF Shuffler:

  • pdftk only supports ASCII filenames. So it’s a bit inconvenient for non-English users like me.
  • PDF Shuffler is way too slow. I tried concatenating several files (approx. 1000 pages), and it kept running for more than 10 minutes before I hit Ctrl-C; pdftk finished the same task in just a few seconds.

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.note.GNU-stack

By chys on December 25th, 2010

GCC always appends one line to any assembler file (.s) file it generates:

	.section	.note.GNU-stack,"",@progbits

Literally, it adds an empty section named .note.GNU-stack to the object file, but it actually serves a hint to the linker* that code in this object file does not require an executable stack. GNU assembler also accepts command-line option “--noexecstack”, which has the same effect.

If every object file contains a section of this name, the linker knows the whole program does not need an executable stack, and the resulting executable will run with a non-executable stack if the OS and underlying hardware support it (see also NX bit).

Why is this important? In practice, virtually no program needs an executable stack (hackers may sometimes use it, though), but buffer overflow attacks frequently insert and run code in stacks. A non-executable stack helps improve security without any overhead.

* GNU linker only.

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Convert PDF to images in Linux

By chys on December 11th, 2010

Just use convert, the universal image converter shipped with ImageMagick.

convert a.pdf a.png

And we get as many PNG files as there are pages in the PDF. They converted files are named a-0.png, a-1.png, …

We can also use it the other way around:

convert a.jpg b.png c.gif abc.pdf

This will combine the three images into one PDF file. Very flexible.

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