Avoid executable path resolution in buildkit-qemu-*

patches/buildkit-direct-execve-v9.2/0008-Avoid-executable-path-resolution.patch

@@ -0,0 +1,93 @@
+From 25cc3455b747b158dd7a328fd1e405a4a2c20ba8 Mon Sep 17 00:00:00 2001
+From: Piotr Stankiewicz <[email protected]>
+Date: Mon, 3 Feb 2025 16:18:28 +0100
+Subject: [PATCH 8/8] Avoid executable path resolution
+
+Signed-off-by: Piotr Stankiewicz <[email protected]>
+---
+ linux-user/main.c           |  7 +++----
+ linux-user/syscall.c        | 12 ++++++------
+ linux-user/user-internals.h |  1 +
+ 3 files changed, 10 insertions(+), 10 deletions(-)
+
+diff --git a/linux-user/main.c b/linux-user/main.c
+index c02535d4e3..acd4c87e68 100644
+--- a/linux-user/main.c
++++ b/linux-user/main.c
+@@ -68,7 +68,7 @@
+ #endif
+
+ char *exec_path;
+-char real_exec_path[PATH_MAX];
++char *real_exec_path;
+
+ static bool opt_one_insn_per_tb;
+ static const char *argv0;
+@@ -825,10 +825,9 @@ int main(int argc, char **argv, char **envp)
+
+     fprintf(stderr, "DEBUG: exec_path=%s after opening execfd\n", exec_path);
+
++    real_exec_path = (char *) malloc(PATH_MAX * sizeof(char));
+     /* Resolve executable file name to full path name */
+-    if (realpath(exec_path, real_exec_path)) {
+-        exec_path = real_exec_path;
+-    }
++    realpath(exec_path, real_exec_path);
+
+     fprintf(stderr, "DEBUG: exec_path=%s after realpath resolution\n", exec_path);
+
+diff --git a/linux-user/syscall.c b/linux-user/syscall.c
+index 63bdb3c154..32753c55bb 100644
+--- a/linux-user/syscall.c
++++ b/linux-user/syscall.c
+@@ -8424,9 +8424,9 @@ static int maybe_do_fake_open(CPUArchState *cpu_env, int dirfd,
+             return -1;
+         }
+         if (safe) {
+-            return safe_openat(dirfd, exec_path, flags, mode);
++            return safe_openat(dirfd, real_exec_path, flags, mode);
+         } else {
+-            return openat(dirfd, exec_path, flags, mode);
++            return openat(dirfd, real_exec_path, flags, mode);
+         }
+     }
+
+@@ -8550,9 +8550,9 @@ ssize_t do_guest_readlink(const char *pathname, char *buf, size_t bufsiz)
+          * Don't worry about sign mismatch as earlier mapping
+          * logic would have thrown a bad address error.
+          */
+-        ret = MIN(strlen(exec_path), bufsiz);
++        ret = MIN(strlen(real_exec_path), bufsiz);
+         /* We cannot NUL terminate the string. */
+-        memcpy(buf, exec_path, ret);
++        memcpy(buf, real_exec_path, ret);
+     } else {
+         ret = readlink(path(pathname), buf, bufsiz);
+     }
+@@ -10599,9 +10599,9 @@ static abi_long do_syscall1(CPUArchState *cpu_env, int num, abi_long arg1,
+                  * Don't worry about sign mismatch as earlier mapping
+                  * logic would have thrown a bad address error.
+                  */
+-                ret = MIN(strlen(exec_path), arg4);
++                ret = MIN(strlen(real_exec_path), arg4);
+                 /* We cannot NUL terminate the string. */
+-                memcpy(p2, exec_path, ret);
++                memcpy(p2, real_exec_path, ret);
+             } else {
+                 ret = get_errno(readlinkat(arg1, path(p), p2, arg4));
+             }
+diff --git a/linux-user/user-internals.h b/linux-user/user-internals.h
+index 46ffc093f4..4b9876b585 100644
+--- a/linux-user/user-internals.h
++++ b/linux-user/user-internals.h
+@@ -24,6 +24,7 @@
+ #include "qemu/log.h"
+
+ extern char *exec_path;
++extern char *real_exec_path;
+ void init_task_state(TaskState *ts);
+ void task_settid(TaskState *);
+ void stop_all_tasks(void);
+-- 
+2.43.0
+

test/change-workdir/chwd.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"bytes"
+	"crypto/md5"
+	"fmt"
+	"os"
+)
+
+func main() {
+	if err := os.Chdir("/tmp"); err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+	if p, err := os.Readlink("/proc/self/exe"); err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	} else {
+		fmt.Println(p)
+	}
+	f, err := os.Open("/proc/self/exe")
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+	defer f.Close()
+	var buf bytes.Buffer
+	buf.ReadFrom(f)
+	hash := md5.Sum(buf.Bytes())
+	f2, err := os.Open(os.Args[1])
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+	buf.Reset()
+	buf.ReadFrom(f2)
+	hash2 := md5.Sum(buf.Bytes())
+	if hash != hash2 {
+		fmt.Printf("/proc/self/exe does not match %s\n", os.Args[1])
+		os.Exit(1)
+	}
+	os.Exit(0)
+}

test/run.sh

@@ -3,6 +3,7 @@
 ln -s /usr/bin/env /work/env
 go build print/printargs.go
 go build exec/execargv0.go
+go build change-workdir/chwd.go
 
 echo "testing $(uname -m)"
 ./test.bats
@@ -18,6 +19,7 @@ fi
 ln -sf /crossarch/usr/bin/env /work/env
 GOARCH=$crossArch go build print/printargs.go
 GOARCH=$crossArch go build exec/execargv0.go
+GOARCH=$crossArch go build change-workdir/chwd.go
 
 if ./printargs >/dev/null 2>/dev/nulll; then
   echo "can't test emulator because $crossEmulator emulator is installed in the kernel"

test/test.bats

@@ -83,12 +83,7 @@ execdirect() {
 @test "shebang-direct" {
   execdirect ./shebang.sh foo bar1
   assert_success
-  if [ -n "$BINFMT_EMULATOR" ]; then
-    # FIXME: direct exec patches should be fixed to not prepend workdir
-    assert_output "./printargs /work/shebang.sh foo bar1"
-  else
-    assert_output "./printargs ./shebang.sh foo bar1"
-  fi
+  assert_output "./printargs ./shebang.sh foo bar1"
 }
 
 @test "relative-exec" {
@@ -140,3 +135,9 @@ execdirect() {
   assert_success
   assert_output "./printargs foo bar1 bar2"
 }
+
+@test "change-workdir-and-read-self-exe" {
+  execdirect ./chwd /work/chwd
+  assert_success
+  assert_output "/work/chwd"
+}