atmel-samd: Enable full urandom module from extmod. #24
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cwalther
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Jun 1, 2024
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 dd376a2 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, adafruit#20] 95ce: 6023 str r3, [r4, #0] 95d0: 6144 str r4, [r0, adafruit#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, adafruit#20] 95de: 681b ldr r3, [r3, #0] 95e0: 6143 str r3, [r0, adafruit#20] 95e2: bd10 pop {r4, pc} 000095e4 <nlr_push>: 95e4: 60c4 str r4, [r0, adafruit#12] 95e6: 6105 str r5, [r0, adafruit#16] 95e8: 6146 str r6, [r0, adafruit#20] 95ea: 6187 str r7, [r0, adafruit#24] 95ec: 4641 mov r1, r8 95ee: 61c1 str r1, [r0, adafruit#28] 95f0: 4649 mov r1, r9 95f2: 6201 str r1, [r0, adafruit#32] 95f4: 4651 mov r1, sl 95f6: 6241 str r1, [r0, adafruit#36] @ 0x24 95f8: 4659 mov r1, fp 95fa: 6281 str r1, [r0, adafruit#40] @ 0x28 95fc: 4669 mov r1, sp 95fe: 62c1 str r1, [r0, adafruit#44] @ 0x2c 9600: 4671 mov r1, lr 9602: 6081 str r1, [r0, adafruit#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>
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This is a small change to the port config to enable the urandom module included in the extmod dir. The full urandom module is enabled so all of its functions are available (the minimum module is too low level and only provides an integer with x random bits, whereas the full module provides the expected python randint, uniform, choice, etc. functions).
The generator just uses a software random number generator so it's pseudorandom numbers, but that's the best the SAMD21 can do. You can seed the generator with a value, perhaps like a random ADC reading (although they don't appear to fluctuate much on feathers) or maybe a time.ticks_ms value.
Also of note there is no explicit alias between urandom and random. IMHO this is fine since this module isn't a complete python random module implementation.
The impact of enabling this module is an increase of about 900 bytes on the firmware size too.