Auto merge of #1975 - skrap:feature/move-uclibc-under-linux, r=JohnTitor

Move uclibc under linux

This is a first cut at moving uclibc under linux, alongside `musl` and `gnu`.

All tests pass on a vexpress a9 running in qemu.  I am working on testing the other tier 3 uclibc targets: `mips-unknown-linux-uclibc` and `mipsel-unknown-linux-uclibc`.  ~Not having access to these platforms, I am working on getting them running in qemu, but it may take a bit.~

The style check doesn't pass.  I would appreciate some direction here.  Many of these constants are defined under 2-of-3 out of musl/glibc/uclibc, so I'm not sure whether I should transform:

```rust
#[cfg(not(target_env = "uclibc"))]
pub fn foo();
```
into 2 separate declarations, one each in `musl/mod.rs` and `gnu/mod.rs` or use a `cfg_if` block for each one (which explodes 2 lines into 5).

- [x] Help me choose which fix to use for the items defined in musl and gnu but not uclibc.

It's my guess that exploding into the `cfg_if` block is better for long-term maintainability, but I'd really appreciate opinions from the maintainers.

Author: bors

libc-test/build.rs

@@ -2267,6 +2267,7 @@ fn test_linux(target: &str) {
     let gnuabihf = target.contains("gnueabihf");
     let x86_64_gnux32 = target.contains("gnux32") && x86_64;
     let riscv64 = target.contains("riscv64");
+    let uclibc = target.contains("uclibc");
 
     let mut cfg = ctest_cfg();
     cfg.define("_GNU_SOURCE", None);
@@ -2377,7 +2378,8 @@ fn test_linux(target: &str) {
                [!(x32 || musl || gnu)]: "sys/sysctl.h",
                // <execinfo.h> is not supported by musl:
                // https://www.openwall.com/lists/musl/2015/04/09/3
-               [!musl]: "execinfo.h",
+               // <execinfo.h> is not present on uclibc.
+               [!(musl || uclibc)]: "execinfo.h",
     }
 
     // Include linux headers at the end:
@@ -2419,16 +2421,18 @@ fn test_linux(target: &str) {
         "linux/sockios.h",
         "linux/vm_sockets.h",
         "linux/wait.h",
-        "sys/auxv.h",
         "sys/fanotify.h",
+        // <sys/auxv.h> is not present on uclibc
+        [!uclibc]: "sys/auxv.h",
     }
 
     // note: aio.h must be included before sys/mount.h
     headers! {
         cfg:
         "sys/xattr.h",
         "sys/sysinfo.h",
-        "aio.h",
+        // AIO is not supported by uclibc:
+        [!uclibc]: "aio.h",
     }
 
     cfg.type_name(move |ty, is_struct, is_union| {
@@ -2651,6 +2655,15 @@ fn test_linux(target: &str) {
             | "CAN_RAW_FILTER_MAX"
             | "CAN_NPROTO" => true,
 
+            "MS_RMT_MASK" if uclibc => true, // updated in glibc 2.22 and musl 1.1.13
+
+            // These are not defined in uclibc but will be passed through to the kernel
+            // so they will be supported if the kernel supports them.  Otherwise the
+            // kernel will return runtime errors.  Since they're required for tokio 
+            // support, we except them from the tests here.
+            // See https://github.com/rust-lang/libc/pull/2019#issuecomment-754351482
+            "EPOLLEXCLUSIVE" | "EPOLLWAKEUP" if uclibc => true,
+
             // FIXME: Requires recent kernel headers (5.8):
             "STATX_MNT_ID" => true,
 
@@ -2698,6 +2711,31 @@ fn test_linux(target: &str) {
             // FIXME: It now takes c_void instead of timezone since glibc 2.31.
             "gettimeofday" if gnu => true,
 
+            // These are all implemented as static inline functions in uclibc, so
+            // they cannot be linked against.
+            // If implementations are required, they might need to be implemented
+            // in this crate.
+            "posix_spawnattr_init" if uclibc => true,
+            "posix_spawnattr_destroy" if uclibc => true,
+            "posix_spawnattr_getsigdefault" if uclibc => true,
+            "posix_spawnattr_setsigdefault" if uclibc => true,
+            "posix_spawnattr_getsigmask" if uclibc => true,
+            "posix_spawnattr_setsigmask" if uclibc => true,
+            "posix_spawnattr_getflags" if uclibc => true,
+            "posix_spawnattr_setflags" if uclibc => true,
+            "posix_spawnattr_getpgroup" if uclibc => true,
+            "posix_spawnattr_setpgroup" if uclibc => true,
+            "posix_spawnattr_getschedpolicy" if uclibc => true,
+            "posix_spawnattr_setschedpolicy" if uclibc => true,
+            "posix_spawnattr_getschedparam" if uclibc => true,
+            "posix_spawnattr_setschedparam" if uclibc => true,
+            "posix_spawn_file_actions_init" if uclibc => true,
+            "posix_spawn_file_actions_destroy" if uclibc => true,
+
+            // uclibc defines the flags type as a uint, but dependent crates
+            // assume it's a int instead.
+            "getnameinfo" if uclibc => true,
+
             _ => false,
         }
     });

src/unix/linux_like/linux/gnu/mod.rs

@@ -287,6 +287,19 @@ s! {
         __re_nsub: ::size_t,
         __bitfield: u8,
     }
+
+    pub struct Elf64_Chdr {
+        pub ch_type: ::Elf64_Word,
+        pub ch_reserved: ::Elf64_Word,
+        pub ch_size: ::Elf64_Xword,
+        pub ch_addralign: ::Elf64_Xword,
+    }
+
+    pub struct Elf32_Chdr {
+        pub ch_type: ::Elf32_Word,
+        pub ch_size: ::Elf32_Word,
+        pub ch_addralign: ::Elf32_Word,
+    }
 }
 
 impl siginfo_t {
@@ -620,6 +633,22 @@ pub const TCP_FASTOPEN: ::c_int = 23;
 pub const TCP_TIMESTAMP: ::c_int = 24;
 pub const TCP_FASTOPEN_CONNECT: ::c_int = 30;
 
+pub const FAN_MARK_INODE: ::c_uint = 0x0000_0000;
+pub const FAN_MARK_MOUNT: ::c_uint = 0x0000_0010;
+// NOTE: FAN_MARK_FILESYSTEM requires Linux Kernel >= 4.20.0
+pub const FAN_MARK_FILESYSTEM: ::c_uint = 0x0000_0100;
+
+pub const AF_IB: ::c_int = 27;
+pub const AF_MPLS: ::c_int = 28;
+pub const AF_NFC: ::c_int = 39;
+pub const AF_VSOCK: ::c_int = 40;
+pub const AF_XDP: ::c_int = 44;
+pub const PF_IB: ::c_int = AF_IB;
+pub const PF_MPLS: ::c_int = AF_MPLS;
+pub const PF_NFC: ::c_int = AF_NFC;
+pub const PF_VSOCK: ::c_int = AF_VSOCK;
+pub const PF_XDP: ::c_int = AF_XDP;
+
 /* DCCP socket options */
 pub const DCCP_SOCKOPT_PACKET_SIZE: ::c_int = 1;
 pub const DCCP_SOCKOPT_SERVICE: ::c_int = 2;
@@ -646,6 +675,7 @@ pub const SIGEV_THREAD_ID: ::c_int = 4;
 pub const BUFSIZ: ::c_uint = 8192;
 pub const TMP_MAX: ::c_uint = 238328;
 pub const FOPEN_MAX: ::c_uint = 16;
+pub const FILENAME_MAX: ::c_uint = 4096;
 pub const POSIX_MADV_DONTNEED: ::c_int = 4;
 pub const _SC_EQUIV_CLASS_MAX: ::c_int = 41;
 pub const _SC_CHARCLASS_NAME_MAX: ::c_int = 45;
@@ -857,11 +887,6 @@ pub const PTRACE_INTERRUPT: ::c_uint = 0x4207;
 pub const PTRACE_LISTEN: ::c_uint = 0x4208;
 pub const PTRACE_PEEKSIGINFO: ::c_uint = 0x4209;
 
-pub const EPOLLWAKEUP: ::c_int = 0x20000000;
-
-pub const SEEK_DATA: ::c_int = 3;
-pub const SEEK_HOLE: ::c_int = 4;
-
 // linux/fs.h
 
 // Flags for preadv2/pwritev2