Linux implements its userland-to-kernel transition using a special library called linux-gate.so that is mapped by the kernel into every process. For more information, see
http://www.trilithium.com/johan/2005/08/linux-gate/
In a nutshell, the problem is that the system call gate function, kernel_vsyscall does not use EBP to point to the frame pointer.
However, the Breakpad processor supports special frames like this via STACK lines in the symbol file. If you look in src/client/linux/data you will see symbol files for linux-gate.so for both Intel & AMD(the implementation of kernel_vsyscall changes depending on the CPU manufacturer). When processing minidumps from Linux 2.6, having these symbol files is necessary for walking the stack for crashes that happen while a thread is in a system call.
If you're just interested in processing minidumps, those two symbol files should be all you need!
The particular details of understanding the linux-gate.so symbol files can be found by reading about STACK lines inside src/common/windows/pdb_source_line_writer.cc, and the above link. To summarize briefly, we just have to inform the processor how to get to the previous frame when the EIP is inside kernel_vsyscall, and we do that by telling the processor how many bytes kernel_vsyscall has pushed onto the stack in it's prologue. For example, one of the symbol files looks somewhat like the following:
MODULE Linux x86 random_debug_id linux-gate.so PUBLIC 400 0 kernel_vsyscall STACK WIN 4 100 1 1 0 0 0 0 0 1
The PUBLIC line indicates that kernel_vsyscall is at offset 400 (in bytes) from the beginning of linux-gate.so. The STACK line indicates the size of the function(100), how many bytes it pushes(1), and how many bytes it pops(1). The last 1 indicates that EBP is pushed onto the stack before being used by the function.
These functions might change significantly depending on kernel version. In my opinion, the actual function stack information is unlikely to change frequently, but the Linux kernel might change the address of kernel_vsyscall w.r.t the beginning of linux-gate.so, which would cause these symbol files to be invalid.