nix develop: Set personality

This makes 'nix develop' set the Linux personality in the same way
that the actual build does, allowing a command like 'nix develop
nix#devShells.i686-linux.default' on x86_64-linux to work correctly.

doc/manual/src/release-notes/rl-next.md

@@ -25,3 +25,9 @@
   $ nix-build glibc^dev`
   ```
   does already.
+
+* On Linux, `nix develop` now sets the
+  [*personality*](https://man7.org/linux/man-pages/man2/personality.2.html)
+  for the development shell in the same way as the actual build of the
+  derivation. This makes shells for `i686-linux` derivations work
+  correctly on `x86_64-linux`.

src/libstore/build/derivation-goal.cc

@@ -39,7 +39,6 @@
 #include <sys/ioctl.h>
 #include <net/if.h>
 #include <netinet/ip.h>
-#include <sys/personality.h>
 #include <sys/mman.h>
 #include <sched.h>
 #include <sys/param.h>

src/libstore/build/local-derivation-goal.cc

@@ -15,6 +15,7 @@
 #include "callback.hh"
 #include "json-utils.hh"
 #include "cgroup.hh"
+#include "personality.hh"
 
 #include <regex>
 #include <queue>
@@ -24,7 +25,6 @@
 #include <termios.h>
 #include <unistd.h>
 #include <sys/mman.h>
-#include <sys/utsname.h>
 #include <sys/resource.h>
 #include <sys/socket.h>
 
@@ -37,7 +37,6 @@
 #include <sys/ioctl.h>
 #include <net/if.h>
 #include <netinet/ip.h>
-#include <sys/personality.h>
 #include <sys/mman.h>
 #include <sched.h>
 #include <sys/param.h>
@@ -1964,33 +1963,7 @@ void LocalDerivationGoal::runChild()
         /* Close all other file descriptors. */
         closeMostFDs({STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO});
 
-#if __linux__
+        setPersonality(drv->platform);
-        /* Change the personality to 32-bit if we're doing an
-           i686-linux build on an x86_64-linux machine. */
-        struct utsname utsbuf;
-        uname(&utsbuf);
-        if ((drv->platform == "i686-linux"
-                && (settings.thisSystem == "x86_64-linux"
-                    || (!strcmp(utsbuf.sysname, "Linux") && !strcmp(utsbuf.machine, "x86_64"))))
-            || drv->platform == "armv7l-linux"
-            || drv->platform == "armv6l-linux")
-        {
-            if (personality(PER_LINUX32) == -1)
-                throw SysError("cannot set 32-bit personality");
-        }
-
-        /* Impersonate a Linux 2.6 machine to get some determinism in
-           builds that depend on the kernel version. */
-        if ((drv->platform == "i686-linux" || drv->platform == "x86_64-linux") && settings.impersonateLinux26) {
-            int cur = personality(0xffffffff);
-            if (cur != -1) personality(cur | 0x0020000 /* == UNAME26 */);
-        }
-
-        /* Disable address space randomization for improved
-           determinism. */
-        int cur = personality(0xffffffff);
-        if (cur != -1) personality(cur | ADDR_NO_RANDOMIZE);
-#endif
 
         /* Disable core dumps by default. */
         struct rlimit limit = { 0, RLIM_INFINITY };

src/libstore/build/personality.cc

@@ -0,0 +1,44 @@
+#include "personality.hh"
+#include "globals.hh"
+
+#if __linux__
+#include <sys/utsname.h>
+#include <sys/personality.h>
+#endif
+
+#include <cstring>
+
+namespace nix {
+
+void setPersonality(std::string_view system)
+{
+#if __linux__
+        /* Change the personality to 32-bit if we're doing an
+           i686-linux build on an x86_64-linux machine. */
+        struct utsname utsbuf;
+        uname(&utsbuf);
+        if ((system == "i686-linux"
+                && (std::string_view(SYSTEM) == "x86_64-linux"
+                    || (!strcmp(utsbuf.sysname, "Linux") && !strcmp(utsbuf.machine, "x86_64"))))
+            || system == "armv7l-linux"
+            || system == "armv6l-linux")
+        {
+            if (personality(PER_LINUX32) == -1)
+                throw SysError("cannot set 32-bit personality");
+        }
+
+        /* Impersonate a Linux 2.6 machine to get some determinism in
+           builds that depend on the kernel version. */
+        if ((system == "i686-linux" || system == "x86_64-linux") && settings.impersonateLinux26) {
+            int cur = personality(0xffffffff);
+            if (cur != -1) personality(cur | 0x0020000 /* == UNAME26 */);
+        }
+
+        /* Disable address space randomization for improved
+           determinism. */
+        int cur = personality(0xffffffff);
+        if (cur != -1) personality(cur | ADDR_NO_RANDOMIZE);
+#endif
+}
+
+}

src/libstore/build/personality.hh

@@ -0,0 +1,11 @@
+#pragma once
+
+#include <string>
+
+namespace nix {
+
+void setPersonality(std::string_view system);
+
+}
+
+

src/nix/develop.cc

@@ -164,6 +164,14 @@ struct BuildEnvironment
     {
         return vars == other.vars && bashFunctions == other.bashFunctions;
     }
+
+    std::string getSystem() const
+    {
+        if (auto v = get(vars, "system"))
+            return getString(*v);
+        else
+            return settings.thisSystem;
+    }
 };
 
 const static std::string getEnvSh =
@@ -570,7 +578,7 @@ struct CmdDevelop : Common, MixEnvironment
             }
         }
 
-        runProgramInStore(store, shell, args);
+        runProgramInStore(store, shell, args, buildEnvironment.getSystem());
     }
 };
 

src/nix/run.cc

@@ -9,6 +9,7 @@
 #include "fs-accessor.hh"
 #include "progress-bar.hh"
 #include "eval.hh"
+#include "build/personality.hh"
 
 #if __linux__
 #include <sys/mount.h>
@@ -24,7 +25,8 @@ namespace nix {
 
 void runProgramInStore(ref<Store> store,
     const std::string & program,
-    const Strings & args)
+    const Strings & args,
+    std::optional<std::string_view> system)
 {
     stopProgressBar();
 
@@ -44,7 +46,7 @@ void runProgramInStore(ref<Store> store,
         throw Error("store '%s' is not a local store so it does not support command execution", store->getUri());
 
     if (store->storeDir != store2->getRealStoreDir()) {
-        Strings helperArgs = { chrootHelperName, store->storeDir, store2->getRealStoreDir(), program };
+        Strings helperArgs = { chrootHelperName, store->storeDir, store2->getRealStoreDir(), std::string(system.value_or("")), program };
         for (auto & arg : args) helperArgs.push_back(arg);
 
         execv(getSelfExe().value_or("nix").c_str(), stringsToCharPtrs(helperArgs).data());
@@ -52,6 +54,9 @@ void runProgramInStore(ref<Store> store,
         throw SysError("could not execute chroot helper");
     }
 
+    if (system)
+        setPersonality(*system);
+
     execvp(program.c_str(), stringsToCharPtrs(args).data());
 
     throw SysError("unable to execute '%s'", program);
@@ -199,6 +204,7 @@ void chrootHelper(int argc, char * * argv)
     int p = 1;
     std::string storeDir = argv[p++];
     std::string realStoreDir = argv[p++];
+    std::string system = argv[p++];
     std::string cmd = argv[p++];
     Strings args;
     while (p < argc)
@@ -262,6 +268,9 @@ void chrootHelper(int argc, char * * argv)
     writeFile("/proc/self/uid_map", fmt("%d %d %d", uid, uid, 1));
     writeFile("/proc/self/gid_map", fmt("%d %d %d", gid, gid, 1));
 
+    if (system != "")
+        setPersonality(system);
+
     execvp(cmd.c_str(), stringsToCharPtrs(args).data());
 
     throw SysError("unable to exec '%s'", cmd);

src/nix/run.hh

@@ -6,6 +6,7 @@ namespace nix {
 
 void runProgramInStore(ref<Store> store,
     const std::string & program,
-    const Strings & args);
+    const Strings & args,
+    std::optional<std::string_view> system = std::nullopt);
 
 }