Does it? Did you forget to add a size check below?

Oh yes, I forgot this check. But as stated in the man page, PTRACE_GETREGS does not return the size of bytes written, so it seems that there is no easy way to check this?

Do you know how C programs use PTRACE_GETREGS for 32-bit targets, then?

As far as I know, we should check the target tracee bitness in advance, instead of checking after the syscall returns. Is it better to mark this method unsafe and encourage users to use getregset for safe tracee register retrieval?

If it cannot guarantee that the return value will be correctly constructed, then yes it must be unsafe. Either that, or it could return a union (which is unsafe to read from).

Updated. I prefer to mark it as unsafe, since it is hard to import the 32bit user_regs_struct in libc crate.

As far as I know, we should check the target tracee bitness in advance, instead of checking after the syscall returns.

From this post, it is not trivial to check the bitness, so I would also prefer to mark this function unsafe as well if we don't have a better option.

since it is hard to import the 32bit user_regs_struct in libc crate

Would you like to elaborate on this a bit?

If we return a union, then this union should involve both 64bit and 32bit version of user_regs_struct, which is provided by libc crate instead of the nix itself. However, the struct definition is bound to the compile target of libc crate, we cannot import a 32bit user_regs_struct in libc if we compiles to x86_64

However, the struct definition is bound to the compile target of libc crate, we cannot import a 32bit user_regs_struct in libc if we compiles to x86_64

Right, the actual type is decided at compile time, so we cannot use the 32-bit version unless we define it in Nix, which is not something Nix should do.

$ pwd
libc/src/unix/linux_like/linux/gnu

$ rg "struct user_regs_struct"
b64/x86_64/mod.rs
178:    pub struct user_regs_struct {

b32/x86/mod.rs
74:    pub struct user_regs_struct {

b64/loongarch64/mod.rs
193:    pub struct user_regs_struct {

b64/riscv64/mod.rs
196:    pub struct user_regs_struct {

b32/riscv32/mod.rs
200:    pub struct user_regs_struct {

b64/aarch64/mod.rs
146:    pub struct user_regs_struct {

As far as I know, we should check the target tracee bitness in advance, instead of checking after the syscall returns.

After figuring out the bitness, how do you use a 32-bit version structure in C if you know the tracee is a 32-bit process? Since in C, these types are also chosen at compile time with #ifdef macros.

In GDB, it is done without the user_regs_struct. The regs are fetched here, and is used here

In short, it provides a 64bit version buffer to receive the regs, and when storing it to internal structure, the tracee arch is checked and collect corresponding register values.