resolve_command: Use faccessat instead of custom permission check

Unix.access uses RUID and RGID, which is not correct when doing a
PATH-search. The faccessat function is able to check permissions using
EUID and EGID instead. opam's hand-rolled check_permissions function is
therefore replaced with a binding for faccessat.

This simultaneously fixes two other things:
- Platforms (such as Cygwin) which use ACLs no longer need special
  support, because their implementations of faccessat already take ACLs
  into account
- We no longer use Unix.getgroups which means that we work around a
  problem with binaries built using musl libc and then used on systems
  where a user belongs to more than 32 groups
  (cf. https://www.openwall.com/lists/musl/2021/07/03/1)

configure.ac

@@ -342,7 +342,7 @@ AS_IF([test "${enable_static}" = yes],[
   ],[
     unix_lib_name=unixnat
   ])
-  echo "(-noautolink -cclib -l${unix_lib_name} -cclib -lmccs_stubs -cclib -lmccs_glpk_stubs -cclib -lsha_stubs ${platform_dependent_stuff})" > src/client/linking.sexp
+  echo "(-noautolink -cclib -l${unix_lib_name} -cclib -lmccs_stubs -cclib -lmccs_glpk_stubs -cclib -lsha_stubs -cclib -lopam_core_stubs ${platform_dependent_stuff})" > src/client/linking.sexp
   AC_MSG_RESULT([static])
 ],[
   AC_MSG_RESULT([shared])

master_changes.md

@@ -272,4 +272,5 @@ users)
   * `OpamHash`: export `compare_kind` [#5561 @rjbou]
   * `OpamFilename`: add `might_escape` to check if a path is escapable, ie contains `<sep>..<sep>` [#5561 @rjbou]
   * Add `OpamStd.Sys.getconf` [#5950 @kit-ty-kate]
+  * `OpamStd.Sys.resolve_command`: Fix opam unable to find executables on systems where users belong to more than 32 groups when opam is built using musl libc [#5381 @kit-ty-kate - fix #5373]
   * `OpamACL`: remove module [#5381 @kit-ty-kate]

src/core/dune

@@ -11,6 +11,14 @@
                (:include ../ocaml-flags-standard.sexp)
                (:include ../ocaml-flags-configure.sexp)
                (:include ../ocaml-context-flags.sexp)))
+  (foreign_stubs
+    (language c)
+    (names opamCommonStubs)
+    (flags :standard
+           -DUNICODE -D_UNICODE -DCAML_NAME_SPACE
+           (:include ../stubs/c-flags.sexp)))
+  (c_library_flags (:standard
+                   (:include c-libraries.sexp)))
   (wrapped     false))
 
 (rule
@@ -41,3 +49,6 @@
   (targets developer)
   (mode    fallback)
   (action  (with-stdout-to %{targets} (echo ""))))
+
+(rule
+  (with-stdout-to c-libraries.sexp (run ocaml %{dep:../../shell/context_flags.ml} clibs)))

src/core/opamCommonStubs.c

@@ -0,0 +1,60 @@
+/**************************************************************************/
+/*                                                                        */
+/*    Copyright 2024 Kate Deplaix                                         */
+/*                                                                        */
+/*  All rights reserved. This file is distributed under the terms of the  */
+/*  GNU Lesser General Public License version 2.1, with the special       */
+/*  exception on linking described in the file LICENSE.                   */
+/*                                                                        */
+/**************************************************************************/
+
+/* Needed for the Windows string conversion functions on older OCaml */
+#define CAML_INTERNALS
+
+#include <caml/mlvalues.h>
+#include <caml/alloc.h>
+#include <caml/memory.h>
+#include <caml/signals.h>
+#include <caml/osdeps.h>
+#include <caml/unixsupport.h>
+#include <caml/version.h>
+
+#ifndef _WIN32
+
+#include <fcntl.h>
+#include <unistd.h>
+
+#else
+
+#include <io.h>
+
+/* mingw-w64 defines R_OK */
+#ifndef R_OK
+#define R_OK 4
+#endif
+
+#endif
+
+#if OCAML_VERSION < 50000
+#define caml_unix_access unix_access
+#endif
+
+CAMLprim value opam_is_executable(value path)
+{
+  CAMLparam1(path);
+  char_os * p;
+  int ret;
+
+  caml_unix_check_path(path, "faccessat");
+  p = caml_stat_strdup_to_os(String_val(path));
+  caml_enter_blocking_section();
+#ifdef _WIN32
+  /* No execute bit on Windows */
+  ret = _waccess(p, R_OK);
+#else
+  ret = faccessat(AT_FDCWD, p, X_OK, AT_EACCESS);
+#endif
+  caml_leave_blocking_section();
+  caml_stat_free(p);
+  CAMLreturn(Val_bool(ret == 0));
+}

src/core/opamStd.ml

@@ -1249,33 +1249,11 @@ module OpamSys = struct
   (* OCaml 4.05.0 no longer follows the updated PATH to resolve commands. This
      makes unqualified commands absolute as a workaround. *)
   let resolve_command =
-    let check_perms =
-      if Sys.win32 then fun f ->
-        try (Unix.stat f).Unix.st_kind = Unix.S_REG
-        with e -> fatal e; false
-      else fun f ->
-        try
-          let {Unix.st_uid; st_gid; st_perm; st_kind; _} = Unix.stat f in
-          if st_kind <> Unix.S_REG then false else
-          let groups =
-            Unix.getegid () :: Array.to_list (Unix.getgroups ())
-          in
-          let mask =
-            if Unix.geteuid () = (st_uid : int) then
-              0o100
-            else if List.mem st_gid groups then
-              0o010
-            else
-              0o001
-          in
-          (st_perm land mask) <> 0
-        with e -> fatal e; false
-    in
     let resolve ?dir env name =
       if not (Filename.is_relative name) then begin
         (* absolute path *)
         if not (Sys.file_exists name) then `Not_found
-        else if not (check_perms name) then `Denied
+        else if not (OpamStubs.is_executable name) then `Denied
         else `Cmd name
       end else if is_external_cmd name then begin
         (* relative path *)
@@ -1284,7 +1262,7 @@ module OpamSys = struct
           | Some d -> Filename.concat d name
         in
         if not (Sys.file_exists cmd) then `Not_found
-        else if not (check_perms cmd) then `Denied
+        else if not (OpamStubs.is_executable cmd) then `Denied
         else `Cmd cmd
       end else
       (* bare command, lookup in PATH *)
@@ -1298,7 +1276,7 @@ module OpamSys = struct
          expected name but not the right permissions are skipped silently.
          Therefore, only two outcomes are possible in that case, [`Cmd ..] or
          [`Not_found]. *)
-      match List.find check_perms possibles with
+      match List.find OpamStubs.is_executable possibles with
       | cmdname -> `Cmd cmdname
       | exception Not_found ->
         if possibles = [] then

src/core/opamStubsTypes.ml

@@ -113,3 +113,8 @@ type win32_version_info = {
   strings: ((int * int) * win32_non_fixed_version_info) list;
     (** Non-fixed string table. First field is a pair of Language and Codepage ID. *)
 }
+
+external is_executable : string -> bool = "opam_is_executable"
+(** faccessat on Unix; _waccess on Windows. Checks whether a path is executable
+    for the current process. On Unix, unlike Unix.access, this is checked using
+    the EUID/EGID rather than RUID/RGID. *)