[{"id":3687290,"web_url":"http://patchwork.ozlabs.org/comment/3687290/","msgid":"<02070874-6c51-4b71-97f2-1312b5a3b231@linaro.org>","list_archive_url":null,"date":"2026-05-06T18:05:24","subject":"Re: [PATCH v5 12/15] target/arm: tag is not a part of PAuth with MTX","submitter":{"id":72104,"url":"http://patchwork.ozlabs.org/api/people/72104/","name":"Richard Henderson","email":"richard.henderson@linaro.org"},"content":"On 5/4/26 10:50, Gabriel Brookman wrote:\n> As described in the section on MTX, tag bits should not be used to store\n> or compute the PAC when MTX is set. See also Authenticate(),\n> InsertPAC(), and Strip().\n> \n> Signed-off-by: Gabriel Brookman \n> ---\n> target/arm/internals.h | 12 +++++++++++-\n> target/arm/tcg/pauth_helper.c | 18 +++++++++++++++++-\n> 2 files changed, 28 insertions(+), 2 deletions(-)\n> \n> diff --git a/target/arm/internals.h b/target/arm/internals.h\n> index d313d36603..31b7e1c85e 100644\n> --- a/target/arm/internals.h\n> +++ b/target/arm/internals.h\n> @@ -1809,7 +1809,17 @@ static inline uint64_t pauth_ptr_mask(ARMVAParameters param)\n> int bot_pac_bit = 64 - param.tsz;\n> int top_pac_bit = 64 - 8 * param.tbi;\n> \n> - return MAKE_64BIT_MASK(bot_pac_bit, top_pac_bit - bot_pac_bit);\n> + uint64_t mask = MAKE_64BIT_MASK(bot_pac_bit, top_pac_bit - bot_pac_bit);\n> +\n> + /*\n> + * If mtx is enabled, second nibble is not part of PAC. See\n> + * InsertPAC().\n> + */\n> + if (param.mtx) {\n> + mask &= ~MAKE_64BIT_MASK(56, 4);\n> + }\n> +\n> + return mask;\n> }\n\nOk.\n\n\n> diff --git a/target/arm/tcg/pauth_helper.c b/target/arm/tcg/pauth_helper.c\n> index 67c0d59d9e..3d83ca4c3c 100644\n> --- a/target/arm/tcg/pauth_helper.c\n> +++ b/target/arm/tcg/pauth_helper.c\n> @@ -342,9 +342,16 @@ static uint64_t pauth_addpac(CPUARMState *env, uint64_t ptr, uint64_t modifier,\n> }\n> \n> /* Build a pointer with known good extension bits. */\n> - top_bit = 64 - 8 * param.tbi;\n> + top_bit = 64 - 8 * (param.tbi || param.mtx);\n> bot_bit = 64 - param.tsz;\n> ext_ptr = deposit64(ptr, bot_bit, top_bit - bot_bit, ext);\n> + /*\n> + * If mtx is active but not tbi, then the top 4 bits are replaced with the\n> + * ext bit, while leaving bits 56-59 alone. See InsertPAC().\n> + */\n> + if (param.mtx && !param.tbi) {\n> + ext_ptr = deposit64(ext_ptr, 60, 4, ext);\n> + }\n> \n> pac = pauth_computepac(env, ext_ptr, modifier, *key);\n> \n> @@ -377,6 +384,11 @@ static uint64_t pauth_addpac(CPUARMState *env, uint64_t ptr, uint64_t modifier,\n> if (param.tbi) {\n> ptr &= ~MAKE_64BIT_MASK(bot_bit, 55 - bot_bit + 1);\n> pac &= MAKE_64BIT_MASK(bot_bit, 54 - bot_bit + 1);\n> + } else if (param.mtx) {\n> + ptr &= ~(MAKE_64BIT_MASK(60, 4) |\n> + MAKE_64BIT_MASK(bot_bit, 55 - bot_bit + 1));\n> + pac &= MAKE_64BIT_MASK(60, 4) |\n> + MAKE_64BIT_MASK(bot_bit, 54 - bot_bit + 1);\n> } else {\n> ptr &= MAKE_64BIT_MASK(0, bot_bit);\n> pac &= ~(MAKE_64BIT_MASK(55, 1) | MAKE_64BIT_MASK(0, bot_bit));\n\nLooks ok, but I wonder if this function should be re-written using pauth_ptr_mask.\n\n> @@ -424,6 +436,10 @@ static uint64_t pauth_auth(CPUARMState *env, uint64_t ptr, uint64_t modifier,\n> cmp_mask = MAKE_64BIT_MASK(bot_bit, top_bit - bot_bit);\n> cmp_mask &= ~MAKE_64BIT_MASK(55, 1);\n> \n> + if (param.mtx) {\n> + cmp_mask &= ~MAKE_64BIT_MASK(56, 4);\n> + }\n\nLikewise.\n\nAnyway,\nReviewed-by: Richard Henderson \n\n\nr~","headers":{"Return-Path":"","X-Original-To":"incoming@patchwork.ozlabs.org","Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=linaro.org header.i=@linaro.org header.a=rsa-sha256\n header.s=google header.b=U3kTCQ/f;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; 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