Le 15/02/2022 à 10:12, Arnd Bergmann a écrit : > On Tue, Feb 15, 2022 at 9:17 AM Ard Biesheuvel <a...@kernel.org> wrote: >> On Mon, 14 Feb 2022 at 17:37, Arnd Bergmann <a...@kernel.org> wrote: >>> From: Arnd Bergmann <a...@arndb.de> >>> >> >> With set_fs() out of the picture, wouldn't it be sufficient to check >> that bit #55 is clear? (the bit that selects between TTBR0 and TTBR1) >> That would also remove the need to strip the tag from the address. >> >> Something like >> >> asm goto("tbnz %0, #55, %2 \n" >> "tbnz %1, #55, %2 \n" >> :: "r"(addr), "r"(addr + size - 1) :: notok); >> return 1; >> notok: >> return 0; >> >> with an additional sanity check on the size which the compiler could >> eliminate for compile-time constant values. > > That should work, but I don't see it as a clear enough advantage to > have a custom implementation. For the constant-size case, it probably > isn't better than a compiler-scheduled comparison against a > constant limit, but it does hurt maintainability when the next person > wants to change the behavior of access_ok() globally. > > If we want to get into micro-optimizing uaccess, I think a better target > would be a CONFIG_CC_HAS_ASM_GOTO_OUTPUT version > of __get_user()/__put_user as we have on x86 and powerpc. >
There is also the user block accesses with user_access_begin()/user_access_end() together with unsafe_put_user() and unsafe_get_user() which allowed us to optimise user accesses on powerpc, especially in the signal code. Christophe