+++ Nick Alcock [06/07/21 20:33 +0100]:
/proc/kallsyms is very useful for tracers and other tools that need to map kernel symbols to addresses. It would be useful if: - there were a mapping between kernel symbol and module name that only changed when the kernel source code is changed. This mapping should not change simply because a module becomes built into the kernel. - there were symbol size information to determine whether an address is within a symbol or outside it, especially given that there could be huge gaps between symbols. Fix this by introducing a new config parameter CONFIG_KALLMODSYMS, which does several things at once. Change scripts/kallsyms.c stdin from "nm" to "nm -S" so that symbol sizes are available. Have sort_symbols() incorporate size info. Emit size info for each symbol into .kallsyms_sizes in the *.s output file. Change module_get_kallsym() to return symbol size as well. Generate a file "modules_thick.builtin" that maps from the thin archives that make up built-in modules to their constituent object files. (This reintroduces the machinery that used to be used to generate modules.builtin. I am not wedded to this mechanism: if someone can figure out a mechanism that does not require recursing over the entire build tree, I'm happy to use it, but I suspect that no such mechanism exists, since the only place the mapping from object file to module exists is in the makefiles themselves. Regardless, this is fairly cheap, adding less than a second to a typical hot-cache build of a large enterprise kernel. This is true even though it needs to be run unconditionally whenever the .config changes.) Generate a linker map ".tmp_vmlinux.map", converting it into ".tmp_vmlinux.ranges", mapping address ranges to object files. Have scripts/kallsyms read these two new files to map symbol addresses to built-in-module names and then write a mapping from object file address to module name to the *.s output file. The mapping consists of three new symbols: - .kallsyms_module_addresses/.kallsyms_module_offsets encodes the address/offset of each object file (derived from the linker map), in exactly the same way as .kallsyms_addresses/.kallsyms_offsets does for symbols. There is no size: instead, the object files are assumed to tile the address space. (This is slightly more space-efficient than using a size). Non-text-section addresses are skipped: for now, all the users of this interface only need module/non-module information for instruction pointer addresses, not absolute-addressed symbols and the like. This restriction can easily be lifted in future. (For why this isn't called .kallsyms_objfiles, see two entries below.) - .kallsyms_module_names encodes the name of each module in a modified form of strtab: notably, if an object file appears in *multiple* modules, all of which are built in, this is encoded via a zero byte, a one-byte module count, then a series of that many null-terminated strings. Object files which appear in only one module in such a multi-module list are redirected to point inside that list, so that modules which contain some object files shared with other modules and some object files exclusive to them do not double up the module name. (There might still be some duplication between multiple multi-module lists, but this is an extremely marginal size effect, and resolving it would require an extra layer of lookup tables which would be even more complex, and incompressible to boot). As a special case, the table starts with a single zero byte which does *not* represent the start of a multi-module list. - .kallsyms_modules connects the two, encoding a table associated 1:1 with .kallsyms_module_addresses / kallsyms_module_offsets, pointing at an offset in .kallsyms_module_names describing which module (or modules, for a multi-module list) the code occupying this address range is part of. If an address range is part of no module (always built-in) it points at 0 (the null byte at the start of the .kallsyms_module_anmes list). Entries in this list that would contain the same value are fused together, along with their corresponding .kallsyms_module_addresses/offsets entries. Due to this fusion process, and because object files can be split apart into multiple parts by the linker for hot/cold partitioning and the like, entries in here do not really correspond to an object file, but more to some contiguous range of addresses which are guaranteed to belong to a single built-in module: so it seems best to call the symbols .kallsyms_modules*. (The generator has a data structure that does correspond more closely to object files, from which .kallsyms_modules is generated, and that does use 'objfiles' terminology.) In kernel/kallsyms, use the new symbol size information in get_symbol_pos(), both to identify the correct symbol and to return correct size information. Emit a new /proc/kallmodsyms file akin to /proc/kallsyms but with built-in-module names and symbol sizes, using a new kallsyms_builtin_module_address() almost identical to kallsyms_sym_address() to get the address corresponding to a given .kallsyms_modules index, and a new get_builtin_module_idx quite similar to get_symbol_pos to determine the index in the .kallsyms_modules array that relates to a given address. Save a little time by exploiting the fact that all callers will only ever traverse this list from start to end by allowing them to pass in the previous index returned from this function as a hint: thus very few bsearches are actually needed. (In theory this could change to just walk straight down kallsyms_module_addresses/offsets and not bother bsearching at all, but doing it this way is hardly any slower and much more robust.) The display process is complicated a little by the weird format of the .kallsyms_module_names table: we have to look for multimodule entries and print them as space-separated lists of module names. The resulting /proc/kallmodsyms file looks like this: ffffffff8b013d20 409 t pt_buffer_setup_aux ffffffff8b014130 11f T intel_pt_interrupt ffffffff8b014250 2d T cpu_emergency_stop_pt ffffffff8b014280 13a t rapl_pmu_event_init [intel_rapl_perf] ffffffff8b0143c0 bb t rapl_event_update [intel_rapl_perf] ffffffff8b014480 10 t rapl_pmu_event_read [intel_rapl_perf] ffffffff8b014490 a3 t rapl_cpu_offline [intel_rapl_perf] ffffffff8b014540 24 t __rapl_event_show [intel_rapl_perf] ffffffff8b014570 f2 t rapl_pmu_event_stop [intel_rapl_perf] This is emitted even if intel_rapl_perf is built into the kernel. Further down, we see what happens when object files are reused by multiple modules, all of which are built in to the kernel: ffffffffa22b3aa0 ab t handle_timestamp [liquidio] ffffffffa22b3b50 4a t free_netbuf [liquidio] ffffffffa22b3ba0 8d t liquidio_ptp_settime [liquidio] ffffffffa22b3c30 b3 t liquidio_ptp_adjfreq [liquidio] [...] ffffffffa22b9490 203 t lio_vf_rep_create [liquidio] ffffffffa22b96a0 16b t lio_vf_rep_destroy [liquidio] ffffffffa22b9810 1f t lio_vf_rep_modinit [liquidio] ffffffffa22b9830 1f t lio_vf_rep_modexit [liquidio] ffffffffa22b9850 d2 t lio_ethtool_get_channels [liquidio] [liquidio_vf] ffffffffa22b9930 9c t lio_ethtool_get_ringparam [liquidio] [liquidio_vf] ffffffffa22b99d0 11 t lio_get_msglevel [liquidio] [liquidio_vf] ffffffffa22b99f0 11 t lio_vf_set_msglevel [liquidio] [liquidio_vf] ffffffffa22b9a10 2b t lio_get_pauseparam [liquidio] [liquidio_vf] ffffffffa22b9a40 738 t lio_get_ethtool_stats [liquidio] [liquidio_vf] ffffffffa22ba180 368 t lio_vf_get_ethtool_stats [liquidio] [liquidio_vf] ffffffffa22ba4f0 37 t lio_get_regs_len [liquidio] [liquidio_vf] ffffffffa22ba530 18 t lio_get_priv_flags [liquidio] [liquidio_vf] ffffffffa22ba550 2e t lio_set_priv_flags [liquidio] [liquidio_vf] ffffffffa22ba580 69 t lio_set_fecparam [liquidio] [liquidio_vf] ffffffffa22ba5f0 92 t lio_get_fecparam [liquidio] [liquidio_vf] [...] ffffffffa22cbd10 175 t liquidio_set_mac [liquidio_vf] ffffffffa22cbe90 ab t handle_timestamp [liquidio_vf] ffffffffa22cbf40 4a t free_netbuf [liquidio_vf] ffffffffa22cbf90 2b t octnet_link_status_change [liquidio_vf] ffffffffa22cbfc0 7e t liquidio_vxlan_port_command.constprop.0 [liquidio_vf] Like /proc/kallsyms, the output is driven by address, so keeps the curious property of /proc/kallsyms that symbols (like free_netbuf above) may appear repeatedly with different addresses: but now, unlike in /proc/kallsyms, we can see that those symbols appear repeatedly because they are *different symbols* that ultimately belong to different modules, all of which are built in to the kernel. Those symbols that come from object files that are genuinely reused and that appear only once in meory get a /proc/kallmodsyms line with [multiple] [modules] on it: consumers will have to be ready to handle such lines. Also, kernel symbols for built-in modules will probably appear interspersed with other symbols that are part of different modules and non-modular always-built-in symbols, which, as usual, have no square-bracketed module denotation. As with /proc/kallsyms, non-root usage produces addresses that are all zero; symbol sizes are treated similarly. The size impact of all of this is minimal: for the case above, the kallsyms2.S file went from 14107772 to 14137245 bytes, a gain of 29743 bytes, or 0.16%: vmlinux gained 10824 bytes, a gain of .017%, and the compressed vmlinux only 7552 bytes, a gain of .08%: though the latter two values are very configuration-dependent, they seem likely to scale roughly with the kernel they are part of. Signed-off-by: Nick Alcock <nick.alcock@xxxxxxxxxx> Signed-off-by: Eugene Loh <eugene.loh@xxxxxxxxxx> Reviewed-by: Kris Van Hees <kris.van.hees@xxxxxxxxxx> --- .gitignore | 1 + Documentation/dontdiff | 1 + Makefile | 21 +- include/linux/module.h | 7 +- init/Kconfig | 8 + kernel/kallsyms.c | 304 ++++++++++++++--- kernel/module.c | 4 +- scripts/Kbuild.include | 6 + scripts/Makefile | 6 + scripts/Makefile.modbuiltin | 56 ++++ scripts/kallsyms.c | 650 +++++++++++++++++++++++++++++++++++- scripts/kconfig/confdata.c | 41 ++- scripts/link-vmlinux.sh | 23 +- scripts/modules_thick.c | 200 +++++++++++ scripts/modules_thick.h | 48 +++ 15 files changed, 1309 insertions(+), 67 deletions(-)
This diffstat is seriously _enormous_. Please don't send patches of this size and expect people to be willing to review :-(. Please break this up into a logical sequence of smaller patches to help your potential reviewers and resend with a cover letter. Thanks, Jessica
