From 7b050b366b7dacfb43779c51702a892d8f1873d0 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?J=2E=20Neusch=C3=A4fer?= 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 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 : 95c4: b510 push {r4, lr} 95c6: 0004 movs r4, r0 95c8: f02d fd42 bl 37050 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 : 95d6: b510 push {r4, lr} 95d8: f02d fd3a bl 37050 95dc: 6943 ldr r3, [r0, #20] 95de: 681b ldr r3, [r3, #0] 95e0: 6143 str r3, [r0, #20] 95e2: bd10 pop {r4, pc} 000095e4 : 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 [3]: https://www.openwall.com/lists/musl/2020/09/25/4 Signed-off-by: J. Neuschäfer --- 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 af2480266bcab..1aa3e06994d3f 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 a22c5df5b9418..e7b24f242bc52 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"