| From 7b050b366b7dacfb43779c51702a892d8f1873d0 Mon Sep 17 00:00:00 2001 |
| From: =?UTF-8?q?J=2E=20Neusch=C3=A4fer?= <j.ne@posteo.net> |
| Date: Sun, 2 Apr 2023 19:58:42 +0200 |
| Subject: [PATCH] py/nlrthumb: Make non-Thumb2 long-jump workaround opt-in. |
| MIME-Version: 1.0 |
| Content-Type: text/plain; charset=UTF-8 |
| Content-Transfer-Encoding: 8bit |
| |
| Although the original motivation given for the workaround[1] is correct, |
| nlr.o and nlrthumb.o are linked with a small enough distance that the |
| problem does not occur, and the workaround isn't necessary. The distance |
| between the b instruction and its target (nlr_push_tail) is just 64 |
| bytes[2], well within the ±2046 byte range addressable by an |
| unconditional branch instruction in Thumb mode. |
| |
| The workaround induces a relocation in the text section (textrel), which |
| isn't supported everywhere, notably not on musl-libc[3], where it causes |
| a crash on start-up. With the workaround removed, micropython works on an |
| ARMv5T Linux system built with musl-libc. |
| |
| This commit changes nlrthumb.c to use a direct jump by default, but |
| leaves the long jump workaround as an option for those cases where it's |
| actually needed. |
| |
| [1]: commit dd376a239dc4f47b0ee7866810fcda151f3cf6dd |
| |
| Author: Damien George <damien.p.george@gmail.com> |
| Date: Fri Sep 1 15:25:29 2017 +1000 |
| |
| py/nlrthumb: Get working again on standard Thumb arch (ie not Thumb2). |
| |
| "b" on Thumb might not be long enough for the jump to nlr_push_tail so |
| it must be done indirectly. |
| |
| [2]: Excerpt from objdump -d micropython: |
| |
| 000095c4 <nlr_push_tail>: |
| 95c4: b510 push {r4, lr} |
| 95c6: 0004 movs r4, r0 |
| 95c8: f02d fd42 bl 37050 <mp_thread_get_state> |
| 95cc: 6943 ldr r3, [r0, #20] |
| 95ce: 6023 str r3, [r4, #0] |
| 95d0: 6144 str r4, [r0, #20] |
| 95d2: 2000 movs r0, #0 |
| 95d4: bd10 pop {r4, pc} |
| |
| 000095d6 <nlr_pop>: |
| 95d6: b510 push {r4, lr} |
| 95d8: f02d fd3a bl 37050 <mp_thread_get_state> |
| 95dc: 6943 ldr r3, [r0, #20] |
| 95de: 681b ldr r3, [r3, #0] |
| 95e0: 6143 str r3, [r0, #20] |
| 95e2: bd10 pop {r4, pc} |
| |
| 000095e4 <nlr_push>: |
| 95e4: 60c4 str r4, [r0, #12] |
| 95e6: 6105 str r5, [r0, #16] |
| 95e8: 6146 str r6, [r0, #20] |
| 95ea: 6187 str r7, [r0, #24] |
| 95ec: 4641 mov r1, r8 |
| 95ee: 61c1 str r1, [r0, #28] |
| 95f0: 4649 mov r1, r9 |
| 95f2: 6201 str r1, [r0, #32] |
| 95f4: 4651 mov r1, sl |
| 95f6: 6241 str r1, [r0, #36] @ 0x24 |
| 95f8: 4659 mov r1, fp |
| 95fa: 6281 str r1, [r0, #40] @ 0x28 |
| 95fc: 4669 mov r1, sp |
| 95fe: 62c1 str r1, [r0, #44] @ 0x2c |
| 9600: 4671 mov r1, lr |
| 9602: 6081 str r1, [r0, #8] |
| 9604: e7de b.n 95c4 <nlr_push_tail> |
| |
| [3]: https://www.openwall.com/lists/musl/2020/09/25/4 |
| |
| Signed-off-by: J. Neuschäfer <j.ne@posteo.net> |
| Upstream: https://github.com/micropython/micropython/commit/7b050b366b7dacfb43779c51702a892d8f1873d0 |
| --- |
| py/mpconfig.h | 6 ++++++ |
| py/nlrthumb.c | 10 +++++++++- |
| 2 files changed, 15 insertions(+), 1 deletion(-) |
| |
| diff --git a/py/mpconfig.h b/py/mpconfig.h |
| index af2480266..1aa3e0699 100644 |
| --- a/py/mpconfig.h |
| +++ b/py/mpconfig.h |
| @@ -587,6 +587,12 @@ |
| /*****************************************************************************/ |
| /* Python internal features */ |
| |
| +// Use a special long jump in nlrthumb.c, which may be necessary if nlr.o and |
| +// nlrthumb.o are linked far apart from each other. |
| +#ifndef MICROPY_NLR_THUMB_USE_LONG_JUMP |
| +#define MICROPY_NLR_THUMB_USE_LONG_JUMP (0) |
| +#endif |
| + |
| // Whether to enable import of external modules |
| // When disabled, only importing of built-in modules is supported |
| // When enabled, a port must implement mp_import_stat (among other things) |
| diff --git a/py/nlrthumb.c b/py/nlrthumb.c |
| index a22c5df5b..e7b24f242 100644 |
| --- a/py/nlrthumb.c |
| +++ b/py/nlrthumb.c |
| @@ -38,6 +38,14 @@ |
| |
| __attribute__((naked)) unsigned int nlr_push(nlr_buf_t *nlr) { |
| |
| + // If you get a linker error here, indicating that a relocation doesn't |
| + // fit, try the following (in that order): |
| + // |
| + // 1. Ensure that nlr.o nlrthumb.o are linked closely together, i.e. |
| + // there aren't too many other files between them in the linker list |
| + // (PY_CORE_O_BASENAME in py/py.mk) |
| + // 2. Set -DMICROPY_NLR_THUMB_USE_LONG_JUMP=1 during the build |
| + // |
| __asm volatile ( |
| "str r4, [r0, #12] \n" // store r4 into nlr_buf |
| "str r5, [r0, #16] \n" // store r5 into nlr_buf |
| @@ -71,7 +79,7 @@ __attribute__((naked)) unsigned int nlr_push(nlr_buf_t *nlr) { |
| "str lr, [r0, #8] \n" // store lr into nlr_buf |
| #endif |
| |
| - #if !defined(__thumb2__) |
| + #if MICROPY_NLR_THUMB_USE_LONG_JUMP |
| "ldr r1, nlr_push_tail_var \n" |
| "bx r1 \n" // do the rest in C |
| ".align 2 \n" |
| -- |
| 2.43.0 |
| |
