Re: /proc/kallsyms and symbol size

From: Paulo Marques
Date: Tue Sep 25 2007 - 12:21:21 EST

Stephane Eranian wrote:

Hi, Stephane

Many monitoring tools use /proc/kallsyms to build a symbol table for the kernel.
This technique has the advantage that it does not require root privileges, nor
an up-to-date /boot/, nor a decompressed kernel in /boot.

The problem is that /proc/kallsyms does not report the size of the symbols.
Yet, the information is available in the kernel as it is used by functions such
as __print_symbol(). Having the size is useful to correlate the address obtained
is a sample with a symbol name. Most tools use an approximation which assumes
symbols are contiguous to estimate the size.

That is actually what the kernel does internally, too. It does not keep the size of the symbol, but tries to guess it from the address of the next non-aliased symbol.

Since the addresses are sorted, this works fine most of the time. This is done to reduce the size used by the symbol table in the running kernel.

Just take a look at "get_symbol_pos" in kernel/kallsyms.c and "get_ksymbol" in kernel/module.c to see exactly how this is done

Paulo Marques -

"There cannot be a crisis today; my schedule is already full."
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