On Thu, Jan 21, 2016 at 2:50 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Thu, 21 Jan 2016 18:19:43 +0100 Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote: > >> Similar to how relative extables are implemented, it is possible to emit >> the kallsyms table in such a way that it contains offsets relative to some >> anchor point in the kernel image rather than absolute addresses. The benefit >> is that such table entries are no longer subject to dynamic relocation when >> the build time and runtime offsets of the kernel image are different. Also, >> on 64-bit architectures, it essentially cuts the size of the address table >> in half since offsets can typically be expressed in 32 bits. >> >> Since it is useful for some architectures (like x86) to retain the ability >> to emit absolute values as well, this patch adds support for both, by >> emitting absolute addresses as positive 32-bit values, and addresses >> relative to the lowest encountered relative symbol as negative values, which >> are subtracted from the runtime address of this base symbol to produce the >> actual address. >> >> Support for the above is enabled by default for all architectures except >> IA-64, whose symbols are too far apart to capture in this manner. > > I'm not really understanding the benefits of this. A smaller address > table is nice, but why is it desirable that "such table entries are no > longer subject to dynamic relocation when the build time and runtime > offsets of the kernel image are different"? IIUC, this means that the relocation work done after decompression now doesn't have to do relocation updates for all these values, which means a smaller relocation table as well. -Kees -- Kees Cook Chrome OS & Brillo Security -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
