The 11/27/2020 13:19, Szabolcs Nagy via Libc-alpha wrote: > This is v2 of > https://sourceware.org/pipermail/libc-alpha/2020-November/119305.html > > To enable BTI support, re-mmap executable segments instead of > mprotecting them in case mprotect is seccomp filtered. > > I would like linux to change to map the main exe with PROT_BTI when > that is marked as BTI compatible. From the linux side i heard the > following concerns about this: > - it's an ABI change so requires some ABI bump. (this is fine with > me, i think glibc does not care about backward compat as nothing > can reasonably rely on the current behaviour, but if we have a > new bit in auxv or similar then we can save one mprotect call.) > - in case we discover compatibility issues with user binaries it's > better if userspace can easily disable BTI (e.g. removing the > mprotect based on some env var, but if kernel adds PROT_BTI and > mprotect is filtered then we have no reliable way to remove that > from executables. this problem already exists for static linked > exes, although admittedly those are less of a compat concern.) > - ideally PROT_BTI would be added via a new syscall that does not > interfere with PROT_EXEC filtering. (this does not conflict with > the current patches: even with a new syscall we need a fallback.) > - solve it in systemd (e.g. turn off the filter, use better filter): > i would prefer not to have aarch64 (or BTI) specific policy in > user code. and there was no satisfying way to do this portably. > > Other concerns about the approach: > - mmap is more expensive than mprotect: in my measurements using > mmap instead of mprotect is 3-8x slower (and after mmap pages > have to be faulted in again), but e.g. the exec time of a program > with 4 deps only increases by < 8% due to the 4 new mmaps. (the > kernel side resource usage may increase too, i didnt look at that.) i tested glibc build time with mprotect vs mmap which should be exec heavy. the real time overhead was < 0.2% on a particular 4 core system with linux 5.3 ubuntu kernel, which i consider to be small. (used PROT_EXEC without PROT_BTI for the measurement). > - _dl_signal_error is not valid from the _dl_process_gnu_property > hook. The v2 set addresses this problem: i could either propagate > the errors up until they can be handled or solve it in the aarch64 > backend by first recording failures and then dealing with them in > _dl_open_check. I choose the latter, but did some refactorings in > _dl_map_object_from_fd that makes the former possible too. > > v2: > - [1/6]: New patch that fixes a missed BTI bug found during v2. > - [2-3/6]: New, _dl_map_object_from_fd failure handling improvements, > these are independent of the rest of the series. > - [4/6]: Move the note handling to a different place (after l_phdr > setup, but before fd is closed). > - [5/6]: Rebased. > - [6/6]: First record errors and only report them later. (this fixes > various failure handling issues.) > > Szabolcs Nagy (6): > aarch64: Fix missing BTI protection from dependencies [BZ #26926] > elf: lose is closely tied to _dl_map_object_from_fd > elf: Fix failure handling in _dl_map_object_from_fd > elf: Move note processing after l_phdr is updated > elf: Pass the fd to note processing > aarch64: Use mmap to add PROT_BTI instead of mprotect [BZ #26831] > > elf/dl-load.c | 110 ++++++++++++++++++++----------------- > elf/rtld.c | 4 +- > sysdeps/aarch64/dl-bti.c | 74 ++++++++++++++++++------- > sysdeps/aarch64/dl-prop.h | 14 +++-- > sysdeps/aarch64/linkmap.h | 2 +- > sysdeps/generic/dl-prop.h | 6 +- > sysdeps/generic/ldsodefs.h | 5 +- > sysdeps/x86/dl-prop.h | 6 +- > 8 files changed, 135 insertions(+), 86 deletions(-) > > -- > 2.17.1 >
