ksymaddr-remote: avoid walking pages

[xairy]

The ksymaddr-remote command invokes Kernel.get_kernel_base(), which calls Kernel.get_maps(), which does pagewalk, which is quite slow in the KGDB mode.

I noticed that the only value ksymaddr-remote actually requires from get_kernel_base() is text_base. Same applies to the kload command and possibly some of the others.

I suspect the text_base value can be figured out without relying on memory maps. E.g., on arm64, VBAR_EL1 points to the vectors symbol, so gef can just calculate the address of _stext based on VBAR_EL1 and kallsyms. I assume for x86, gef could rely on the relative address of the symbol for div0_handler. And there should be something similar for stvec on RISCV.

So it would be great to split out the text_base lookup functionality from get_kernel_base, make it work without relying on pagewalking, and then use it in ksymaddr-remote (and kload and others).

Also, a few related changes that could be done:

• I think it makes sense to rename get_kernel_base to e.g. get_kernel_bases or get_kernel_layout to point out that it's not just about finding out a single base address.

• I see that get_kernel_base() relies on sysreg --exact VBAR to get the VBAR_EL1 register value for arm64. This initially failed for me, as my custom GDB extension named the register with its proper name, VBAR_EL1. I see that all the other gef code uses get_register("$VBAR") or get_register("$VBAR_EL1") to get the VBAR value (I assume to support both arm32 and arm64). So using this approach (i.e. also checking for VBAR_EL1) in get_kernel_base() seems like a worthy change to me.