Messages in this thread | | | Date | Wed, 14 Nov 2018 16:15:37 +0100 | From | Jessica Yu <> | Subject | Re: [PATCH v3] ARM: module: Fix function kallsyms on Thumb-2 |
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+++ Dave Martin [13/11/18 13:57 +0000]: >On Tue, Nov 13, 2018 at 12:27:45PM +0100, Vincent Whitchurch wrote: >> Thumb-2 functions have the lowest bit set in the symbol value in the >> symtab. When kallsyms are generated for the vmlinux, the kallsyms are >> generated from the output of nm, and nm clears the lowest bit. >> >> $ arm-linux-gnueabihf-readelf -a vmlinux | grep show_interrupts >> 95947: 8015dc89 686 FUNC GLOBAL DEFAULT 2 show_interrupts >> $ arm-linux-gnueabihf-nm vmlinux | grep show_interrupts >> 8015dc88 T show_interrupts >> $ cat /proc/kallsyms | grep show_interrupts >> 8015dc88 T show_interrupts >> >> However, for modules, the kallsyms uses the values in the symbol table >> without modification, so for functions in modules, the lowest bit is set >> in kallsyms. >> >> $ arm-linux-gnueabihf-readelf -a drivers/net/tun.ko | grep tun_get_socket >> 268: 000000e1 44 FUNC GLOBAL DEFAULT 2 tun_get_socket >> $ arm-linux-gnueabihf-nm drivers/net/tun.ko | grep tun_get_socket >> 000000e0 T tun_get_socket >> $ cat /proc/kallsyms | grep tun_get_socket >> 7fcd30e1 t tun_get_socket [tun] >> >> Because of this, the offset of the crashing instruction shown in oopses >> is incorrect when the crash is in a module. For example, given a >> tun_get_socket which starts like this, >> >> 000000e0 <tun_get_socket>: >> e0: b500 push {lr} >> e2: f7ff fffe bl 0 <__gnu_mcount_nc> >> e6: 4b08 ldr r3, [pc, #32] >> e8: 6942 ldr r2, [r0, #20] >> ea: 429a cmp r2, r3 >> ec: d002 beq.n f4 <tun_get_socket+0x14> >> >> a crash when tun_get_socket is called with NULL results in: >> >> PC is at tun_get_socket+0x7/0x2c [tun] >> pc : [<7fcdb0e8>] >> >> which can result in the incorrect line being reported by gdb if this >> symbol+offset is used there. If the crash is on the first instruction >> of a function, the "PC is at" line would also report the symbol name of >> the preceding function. >> >> To solve this, introduce a weak module_kallsyms_symbol_value() function >> which arches can override to fix up these symbol values, and implement > >(Jumping into this patch without having reviewed previous versions in >detail, so I may have misunderstood some things...) > > >Anyway: > >I prefer this to the previous approach of directly hacking the symbol >values in the module kallsyms table. > >> this for Thumb-2. We need to move elf_type() to st_other so that the >> original value of st_info is preserved. > >Are you sure modifying st_other won't break anything? > >It's hard to imagine how ELF symbol visibility would be relevant in the >kernel, but some arches put other stuff in st_other. See alpha, >powerpc, sh for example. I haven't attempted to understand whether any >of those uses matters here.
Yeah, the st_other field is used to apply relocations in those arches you mention. Overwriting st_info/st_other only "works" here in the sense that that field is no longer needed after applying relocations (add_kallsyms() is called in post_relocation() in the module loader). I agree it's hacky to reuse the field in that way, though.
>Ideally, we wouldn't abuse st_info to store elf_type() in the first >place, but that may be messier to solve. kallsyms could wrap the >ElfXX_Sym in another struct to store extra metadata for example, but it >would increase runtime memory consumption.
Yeah, I've always thought that was ugly. I think it was done as a small optimization for kallsyms, so that we're not looking up what char to print for every symbol, so the st_info field was repurposed for this.
>Another option would be wedge STT_FUNC in bit 7 of st_info, say. >Since elf_type() always yields an ASCII char, that bit shouldn't be >used for anything today. We would of course need to wrap further >access to st_info to mask that bit off as appropriate.
Hm, actually on second thought, I don't think the st_size field is used *anywhere* in the module loader, not in the arch-specific relocation code nor in kallsyms. Perhaps that field could be used to store the output of elf_type instead. Then we won't run into any issues with delayed relocations with livepatch, as the st_size field isn't used at all for applying relocs anyway. Since this field is not used at runtime, I think we can use st_size instead of st_other or st_info, which are two fields that some arches do use to apply relocs. Would be great if someone could confirm/fact-check me on this.
Thanks!
Jessica
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