Rust apps hit SIGSEGV randomly during syscalls, regression in Rust 1.71.0 and newer

[BarryBo]

In MUSL, __syscall_cp_c() polls for __pthread_self()->cancel not equal to zero, and if true, calls __cancel() to stop the thread. This code is all correct, but when Rust apps make a syscall, either the __pthread_self() pointer is bad, or the value of ->cancel is bad (0x10000 typeically), and the attempt to cancel the sole thread crashes with SIGSEGV.

This is only a problem for Rust apps built with Rust 1.71.0 or newer, Older builds of rustc don't repro the issue. I'll investigate.

Here is a typical callstack from a SIGSEGV during a syscall for open() in a single-threaded app that isn't using pthreads for anything:

(gdb) info stack
#0 0x00000000 in ?? ()
#1 0xbeec564e in pthread_exit () from target:/lib/ld-musl-armhf.so.1
#2 0xbeed70fa in ?? () from target:/lib/ld-musl-armhf.so.1
#3 0xbeed7156 in ?? () from target:/lib/ld-musl-armhf.so.1
#4 0xbeeaa3ca in open64 () from target:/lib/ld-musl-armhf.so.1
#5 0xbeeaa248 in __z_azsp_open () from target:/lib/ld-musl-armhf.so.1
#6 0x0002f916 in ADC_Open (id=0) at /opt/azurespheresdk/Sysroots/16/usr/include/applibs/adc_internal.h:24
#7 0x0003044c in ADC_Open_inline (id=0) at ../azure-sphere-sys/static_inline_helpers.c:30
#8 0x0001c4d8 in azure_sphere::applibs::adc::AdcController::new (controller_id=0)
at azure-sphere/src/applibs/adc.rs:63
#9 0x00016ca0 in allapis::main () at samples/allapis/src/main.rs:62

[BarryBo]

Rust 1.71.0 contains this commit: rust-lang/rust@4af36a4 "Upgrade to musl 1.2.3" the previous was musl 1.1.24.