Fix inconsistent reg struct layout for 64bit tracer and 32bit tracee

[Evian-Zhang]

What does this PR do

Fix #2447

Checklist:

• [ x] I have read CONTRIBUTING.md
• [ x] I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[asomers]

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

[Evian-Zhang]

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?

[asomers]

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

[Evian-Zhang]

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?

[asomers]

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).

[Evian-Zhang]

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

[SteveLauC]

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?

[Evian-Zhang]

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

[SteveLauC]

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.

[Evian-Zhang]

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.

[asomers]

Could you please add a changelog entry? Also, have you audited the other ptrace functions for similar behavior?

[Evian-Zhang]

Could you please add a changelog entry? Also, have you audited the other ptrace functions for similar behavior?

I'll add one. For other ptrace functions, it is the setregs and setregset that should be considered. It does not involves UB in Rust side (no uninitialized memory is read by Rust), but it is clear that in the 64bit tracer and 32bit tracee situation, those functions does not do the right thing, and even worse, it may modify the tracee's regs with wrong data. Should we mark it unsafe or add some docs about this?

[asomers]

What about the ptrace functions in the BSD module? Did you look at them?

[Evian-Zhang]

What about the ptrace functions in the BSD module? Did you look at them?

I do not have a BSD machine, and cannot check whether it has the same behavior with Linux