Hi, Willy
On Wed, Jun 28, 2023 at 09:39:56PM +0800, Zhangjin Wu wrote:
To support syscalls (e.g. mmap()) who return a pointer and to allow the pointer as big as possible, we should convert the negated errno value to unsigned long (uintptr_t), otherwise, in signed long, a potential big pointer (whose highest bit is 1) will be treated as a failure.
tools/include/nolibc/errno.h defines the MAX_ERRNO, let's use it directly.
It might or might not work, it's an ABI change that, if validated, at least needs a much more detailed explanation. What matters is not what errno values we're willing to consider as an error, but what the *syscalls* themselves return as an error. If a syscall says "< 0 is an error equal to -errno", it means that we must treat it as an error, and extract its value to get errno. If that errno is larger than MAX_ERRNO it just means we don't know what the error is.
Yes, we do need to find a 'spec' or 'standard' to follow.
welcome suggestions from Arnd, Thomas and also David.
Syscalls that return pointer use that -MAX_ERRNO range to encode errors (such as mmap()). I just do not know if there is a convention saying that other ones also restrict themselves to that range or not. If you find some info which guarantees that it's the case for all of them, then by all means let's proceed like this, but in this case it should be mentioned in the comment why we think it's valid to do this. For now it's presented as an opportunity only.
Currently, I only found a prove-in-use case in musl:
https://elixir.bootlin.com/musl/latest/source/src/internal/syscall_ret.c:
#include <errno.h> #include "syscall.h"
long __syscall_ret(unsigned long r) { if (r > -4096UL) { errno = -r; return -1; } return r; }
Our new implementation (based on the one used by mmap()) is almostly the same as musl. Not sure if this is enough. I have tried to 'git blame' on __syscall_ret() of musl to find some clue, but failed, because the function has been added before importing into its git repo.
Also, the rest of the commit message regarding uintptr_t (which we don't use), bit values and modular arithmetics is extremely confusing and not needed at all. What matters is only to know if we need to consider only values -MAX_ERRNO..-1 as error or all negative ones. If so, then it's obvious that ret >= (unsigned long)-MAX_ERRNO catches them all, as the current mmap() function already does with -4095UL.
Yes, will clean up the commit message, but at first, let's continue get more information about which one is ok:
- -MAX_ERRNO..-1 as error, for sys_mmap (we know in nolibc) currently
- all negative ones, for others currently
I just don't know where to check if we can generalize that test. In the worst case we could have two __sys_ret(), the current one and a second one for pointers. But I would suspect we could generalize due to ptrace, as there it makes sense to be able to detect failures, even unknown ones. I just need something more convincing than an intuition for a commit message and to take such a change :-/
Of course, must be clear enough.
Best regards, Zhangjin
Thanks! Willy