[musl] errno and swapcontext in a multithreaded setup

["Andrey Bugaevskiy" <bugaevskiy@yandex-team.ru>]

Hi,

I'm using makecontext/swapcontext to migrate contexts between threads
and this sometimes leads to getting incorrect errno values.

Investigating further I've noticed that __errno_location
is marked __attribute__((const)).
This causes optimizers to assume that errno address never changes
in the scope of the function which is not the case in my scenario.

Namely, this code:

int test(ucontext_t* old_ctx, const ucontext_t* new_ctx) {
    int err_before = errno;
    swapcontext(old_ctx, new_ctx);
    int err_after = errno;
    return err_before | err_after; // do not optimize out
}

translates with -O1 to something like this:

0000000000001109 <test>:
1109: endbr64
110d: push %r13
110f: push %r12
1111: push %rbp
1112: push %rbx
1113: sub $0x8,%rsp
1117: mov %rdi,%r12
111a: mov %rsi,%r13
111d: callq 1030 <__errno_location@plt>
1122: mov %rax,%rbx
1125: mov (%rax),%ebp
1127: mov %r13,%rsi
112a: mov %r12,%rdi
112d: callq 1020 <swapcontext@plt>
1132: mov %ebp,%eax
1134: or (%rbx),%eax
1136: add $0x8,%rsp
113a: pop %rbx
113b: pop %rbp
113c: pop %r12
113e: pop %r13
1140: retq

errno location is being stored to a register and then reused.
However a call to __errno_location after swapcontext is expected to
return a different address if the context have been swapped back
into another thread.
There are a couple of similarly affected functions (pthread_self,
__h_errno_location).

Removing __attribute__((const)) or changing it
to __attribute__((pure)) resolves the problem in newly compiled code.

Can this change be considered for the future versions of musl?

-- 
Andrey Bugaevskiy