linux-user: Fix AT_EXECFN in AUXV for symlinked programs

[PATCH] linux-user: Fix AT_EXECFN in AUXV for symlinked programs

Posted by Helge Deller 4 weeks, 1 day ago

From: Helge Deller <deller@gmx.de>

The AT_EXECFN entry in AUXV needs to keep the value which was used when
the program was started. Especially for symlinked programs qemu should
not try to resolve the realpath.

Here is a reproducer:
(arm64-chroot)root@p100:/# cd /usr/bin
(arm64-chroot)root@p100:/usr/bin# ln -s echo testprog
(arm64-chroot)root@p100:/usr/bin# LD_SHOW_AUXV=1 ./testprog | grep AT_EXECFN
AT_EXECFN:            ./testprog

In this example, "./testprog" is the correct output, and not "/usr/bin/echo".

This patch fixes parts of commit 258bec39 ("linux-user: Fix access to
/proc/self/exe").

Fixes: 258bec39 ("linux-user: Fix access to /proc/self/exe")
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3379
Signed-off-by: Helge Deller <deller@gmx.de>
---
 linux-user/main.c           |  6 ++++--
 linux-user/syscall.c        | 14 +++++++-------
 linux-user/user-internals.h |  1 +
 3 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/linux-user/main.c b/linux-user/main.c
index 86d04cca3c..c08c73fd80 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -772,8 +772,10 @@ int main(int argc, char **argv, char **envp)
     }
 
     /* Resolve executable file name to full path name */
-    if (realpath(exec_path, real_exec_path)) {
-        exec_path = real_exec_path;
+    /* Keep how we started the program in exec_path, e.g. "./my_program" */
+    /* Store real path in real_exec_path, e.g. "/usr/local/bin/my_program" */
+    if (!realpath(exec_path, real_exec_path)) {
+        printf("Could not resolve %s\n", exec_path);
     }
 
     /*
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index d3d9fffb54..65bbeb8551 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -8790,9 +8790,9 @@ 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);
         }
     }
 
@@ -8929,9 +8929,9 @@ 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);
     }
@@ -9022,7 +9022,7 @@ static int do_execv(CPUArchState *cpu_env, int dirfd,
 
     const char *exe = p;
     if (is_proc_myself(p, "exe")) {
-        exe = exec_path;
+        exe = real_exec_path;
     }
     ret = is_execveat
         ? safe_execveat(dirfd, exe, argp, envp, flags)
@@ -11033,9 +11033,9 @@ 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 24d35998f0..7730444aa5 100644
--- a/linux-user/user-internals.h
+++ b/linux-user/user-internals.h
@@ -24,6 +24,7 @@
 #include "exec/translation-block.h"
 
 extern char *exec_path;
+extern char real_exec_path[PATH_MAX];
 void init_task_state(TaskState *ts);
 void task_settid(TaskState *);
 void stop_all_tasks(void);
-- 
2.53.0

Re: [PATCH] linux-user: Fix AT_EXECFN in AUXV for symlinked programs

Posted by Michael Tokarev 1 week, 3 days ago

On 01.05.2026 14:18, Helge Deller wrote:
> From: Helge Deller <deller@gmx.de>
> 
> The AT_EXECFN entry in AUXV needs to keep the value which was used when
> the program was started. Especially for symlinked programs qemu should
> not try to resolve the realpath.
> 
> Here is a reproducer:
> (arm64-chroot)root@p100:/# cd /usr/bin
> (arm64-chroot)root@p100:/usr/bin# ln -s echo testprog
> (arm64-chroot)root@p100:/usr/bin# LD_SHOW_AUXV=1 ./testprog | grep AT_EXECFN
> AT_EXECFN:            ./testprog
> 
> In this example, "./testprog" is the correct output, and not "/usr/bin/echo".
> 
> This patch fixes parts of commit 258bec39 ("linux-user: Fix access to
> /proc/self/exe").
> 
> Fixes: 258bec39 ("linux-user: Fix access to /proc/self/exe")
> Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3379
> Signed-off-by: Helge Deller <deller@gmx.de>

> --- a/linux-user/user-internals.h
> +++ b/linux-user/user-internals.h
> @@ -24,6 +24,7 @@
>   #include "exec/translation-block.h"
>   
>   extern char *exec_path;
> +extern char real_exec_path[PATH_MAX];

btw, why this is not char*, allocated dynamically?

I'm picking this up for qemu-stable.  Please let me know if I shouldn't.

Thanks,

/mjt

Re: [PATCH] linux-user: Fix AT_EXECFN in AUXV for symlinked programs

Posted by Helge Deller 1 week, 3 days ago

On 5/19/26 22:55, Michael Tokarev wrote:
> On 01.05.2026 14:18, Helge Deller wrote:
>> From: Helge Deller <deller@gmx.de>
>>
>> The AT_EXECFN entry in AUXV needs to keep the value which was used when
>> the program was started. Especially for symlinked programs qemu should
>> not try to resolve the realpath.
>>
>> Here is a reproducer:
>> (arm64-chroot)root@p100:/# cd /usr/bin
>> (arm64-chroot)root@p100:/usr/bin# ln -s echo testprog
>> (arm64-chroot)root@p100:/usr/bin# LD_SHOW_AUXV=1 ./testprog | grep AT_EXECFN
>> AT_EXECFN:            ./testprog
>>
>> In this example, "./testprog" is the correct output, and not "/usr/bin/echo".
>>
>> This patch fixes parts of commit 258bec39 ("linux-user: Fix access to
>> /proc/self/exe").
>>
>> Fixes: 258bec39 ("linux-user: Fix access to /proc/self/exe")
>> Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3379
>> Signed-off-by: Helge Deller <deller@gmx.de>
> 
>> --- a/linux-user/user-internals.h
>> +++ b/linux-user/user-internals.h
>> @@ -24,6 +24,7 @@
>>   #include "exec/translation-block.h"
>>   extern char *exec_path;
>> +extern char real_exec_path[PATH_MAX];
> 
> btw, why this is not char*, allocated dynamically?

no specific reason.
  
> I'm picking this up for qemu-stable.  Please let me know if I shouldn't.

Please pick up.

Thanks!
Helge