Ard Biesheuvel <ardb+git@xxxxxxxxxx> writes: > From: Ard Biesheuvel <ardb@xxxxxxxxxx> > > The kexec purgatory is built like a kernel module, i.e., a partially > linked ELF object where each section is allocated and placed > individually, and all relocations need to be fixed up, even place > relative ones. > > This makes sense for kernel modules, which share the address space with > the core kernel, and contain unresolved references that need to be wired > up to symbols in other modules or the kernel itself. > > The purgatory, however, is a fully linked binary without any external > references, or any overlap with the kernel's virtual address space. So > it makes much more sense to create a fully linked ELF executable that > can just be loaded and run anywhere in memory. It does have external references that are resolved when it is loaded. Further it is at least my impression that non-PIC code is more efficient. PIC typically requires silly things like Global Offset Tables that non-PIC code does not. At first glance this looks like a code passivization. Now at lot of functionality has been stripped out of purgatory so maybe in it's stripped down this make sense, but I want to challenge the notion that this is the obvious thing to do. > The purgatory build on x86 has already switched over to position > independent codegen, which only leaves a handful of absolute references, > which can either be dropped (patch #3) or converted into a RIP-relative > one (patch #4). That leaves a purgatory executable that can run at any > offset in memory with applying any relocations whatsoever. I missed that conversation. Do you happen to have a pointer? I would think the 32bit code is where the PIC would be most costly as the 32bit x86 instruction set predates PIC being a common compilation target. > Some tweaks are needed to deal with the difference between partially > (ET_REL) and fully (ET_DYN/ET_EXEC) linked ELF objects, but with those > in place, a substantial amount of complicated ELF allocation, placement > and patching/relocation code can simply be dropped. Really? As I recall it only needed to handle a single allocation type, and there were good reasons (at least when I wrote it) to patch symbols. Again maybe the fact that people have removed 90% of the functionality makes this make sense, but that is not obvious at first glance. > The last patch in the series removes this code from the generic kexec > implementation, but this can only be done once other architectures apply > the same changes proposed here for x86 (powerpc, s390 and riscv all > implement the purgatory using the shared logic) > > Link: https://lore.kernel.org/all/CAKwvOd=3Jrzju++=Ve61=ZdeshxUM=K3-bGMNREnGOQgNw=aag@xxxxxxxxxxxxxx/ > Link: https://lore.kernel.org/all/20240418201705.3673200-2-ardb+git@xxxxxxxxxx/ > > Cc: Arnd Bergmann <arnd@xxxxxxxx> > Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx> > Cc: kexec@xxxxxxxxxxxxxxxxxxx > Cc: Nathan Chancellor <nathan@xxxxxxxxxx> > Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > Cc: Bill Wendling <morbo@xxxxxxxxxx> > Cc: Justin Stitt <justinstitt@xxxxxxxxxx> > Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx> > > Ard Biesheuvel (9): > x86/purgatory: Drop function entry padding from purgatory > x86/purgatory: Simplify stack handling > x86/purgatory: Drop pointless GDT switch > x86/purgatory: Avoid absolute reference to GDT > x86/purgatory: Simplify GDT and drop data segment > kexec: Add support for fully linked purgatory executables > x86/purgatory: Use fully linked PIE ELF executable > x86/purgatory: Simplify references to regs array > kexec: Drop support for partially linked purgatory executables > > arch/x86/include/asm/kexec.h | 8 - > arch/x86/kernel/kexec-bzimage64.c | 8 - > arch/x86/kernel/machine_kexec_64.c | 127 ---------- > arch/x86/purgatory/Makefile | 17 +- > arch/x86/purgatory/entry64.S | 96 ++++---- > arch/x86/purgatory/setup-x86_64.S | 31 +-- > arch/x86/purgatory/stack.S | 18 -- > include/asm-generic/purgatory.lds | 34 +++ > kernel/kexec_file.c | 255 +++----------------- > 9 files changed, 125 insertions(+), 469 deletions(-) > delete mode 100644 arch/x86/purgatory/stack.S > create mode 100644 include/asm-generic/purgatory.lds Eric _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
