[Crash-utility] relocatable FC7/upstream x86 kernels (was: crash-4.0-4.3 and linux-2.6.22.1-20.fc7)

In the "heading-down-a-slippery-slope" department, I've hacked up a version of crash that is capable of dealing with the relocatable x86 FC7/upstream kernels, whose kernel symbol values in the vmlinux file do not match up with their counterparts when the kernel is actually loaded. In the "vmlinux and /dev/crash do not match" FC7/upstream scenario, the kernel gets compiled with: CONFIG_PHYSICAL_START=0x1000000 (16MB) CONFIG_PHYSICAL_ALIGN=0x400000 (4MB) In that case, the kernel symbols start at PAGE_OFFSET (c0000000) plus the CONFIG_PHYSICAL_START value, or c1000000. However, despite its name of "CONFIG_PHYSICAL_START", the kernel is actually loaded at 4MB physical, so the real "physical start" location looks to be controlled by the CONFIG_PHYSICAL_ALIGN value. (Vivek, correct me if I'm wrong...) So, whereas the vmlinux file shows these symbol values: $ nm -Bn vmlinux ... c1000000 T _text c1000000 T startup_32 c1001000 T startup_32_smp c1001080 t checkCPUtype c1001101 t is486 c1001108 t is386 c1001175 t check_x87 c10011a0 T setup_pda c10011c2 t setup_idt c10011df t rp_sidt c1001262 t early_divide_err c1001268 t early_illegal_opcode c1001271 t early_protection_fault c1001278 t early_page_fault c100127f t early_fault c10012a7 t hlt_loop c10012ac t ignore_int c10012f0 T _stext c10012f0 t run_init_process c10012f0 T stext c1001304 t init_post ... But when loaded into memory, they are all changed to reflect that the kernel was loaded at at 4MB physical instead of 16MB: $ cat /proc/kallsyms c0400000 T _text c0400000 T startup_32 c0401000 T startup_32_smp c0401080 t checkCPUtype c0401101 t is486 c0401108 t is386 c0401175 t check_x87 c04011a0 T setup_pda c04011c2 t setup_idt c04011df t rp_sidt c0401262 t early_divide_err c0401268 t early_illegal_opcode c0401271 t early_protection_fault c0401278 t early_page_fault c040127f t early_fault c04012a7 t hlt_loop c04012ac t ignore_int c04012f0 T _stext c04012f0 t run_init_process c04012f0 T stext c0401304 t init_post ... So in the case above, it amounts to a 12MB relocation from from the compiled-in value to the loaded value. And so if I: (1) hack in the relocation value when reading/storing the vmlinux symbols, and later on (2) back-patch all of the "incorrect" symbols stored by gdb from the vmlinux file -- in the same manner as when a System.map file used, then crash comes up fine, and everything seems to work OK. (Although, as is the case when a System.map file is used to back-patch gdb's notion of symbol values, line numbers from gdb are unavailable) Anyway, I can't find anything obvious in the vmlinux file that indicates what the relocation value would be. On a live system, the vmlinux symbols can be matched with /proc/kallsyms if it exists. If /proc/kallsyms doesn't exist, or if running against a dumpfile, the only option I can think of is adding a crash command line "relocation" argument. On the other hand, it's preferable to configure the kernel such that the virtual address for which it is compiled results in "unity-mapped" kernel virtual addresses. That has always been the case, where the kernel is compiled with a base virtual address of c0100000 or c0400000, gets loaded at a base physical address of 1MB or 4MB respectively, so that a virtual-to- physical translation can be done by subtracting the c0000000 (PAGE_OFFSET) unity-map identifier. To make that happen with the FC7/upstream kernels, the CONFIG_PHYSICAL_START address needs to be equal to or less than the CONFIG_PHYSICAL_ALIGN value. In other words, I've rebuilt with these two combinations: CONFIG_PHYSICAL_START=0x100000 (1MB) CONFIG_PHYSICAL_ALIGN=0x400000 (4MB) or CONFIG_PHYSICAL_START=0x400000 (4MB) CONFIG_PHYSICAL_ALIGN=0x400000 (4MB) and in both cases the kernel gets compiled for c0400000 as a base virtual address. In any case, there is hope for handling such kernels. Dave