Re: [PATCH v3] linux-user/syscall: Implement execve without execveat

[Pierrick Bouvier <pierrick.bouvier@linaro.org>]

On 7/5/23 17:46, Richard Henderson wrote:
> On 7/5/23 14:10, Pierrick Bouvier wrote:
>> Support for execveat syscall was implemented in 55bbe4 and is available
>> since QEMU 8.0.0. It relies on host execveat, which is widely available
>> on most of Linux kernels today.
>>
>> However, this change breaks qemu-user self emulation, if "host" qemu
>> version is less than 8.0.0. Indeed, it does not implement yet execveat.
>> This strange use case happens with most of distribution today having
>> binfmt support.
>>
>> With a concrete failing example:
>> $ qemu-x86_64-7.2 qemu-x86_64-8.0 /bin/bash -c /bin/ls
>> /bin/bash: line 1: /bin/ls: Function not implemented
>> -> not implemented means execve returned ENOSYS
>>
>> qemu-user-static 7.2 and 8.0 can be conveniently grabbed from debian
>> packages qemu-user-static* [1].
>>
>> One usage of this is running wine-arm64 from linux-x64 (details [2]).
>> This is by updating qemu embedded in docker image that we ran into this
>> issue.
>>
>> The solution to update host qemu is not always possible. Either it's
>> complicated or ask you to recompile it, or simply is not accessible
>> (GitLab CI, GitHub Actions). Thus, it could be worth to implement execve
>> without relying on execveat, which is the goal of this patch.
>>
>> This patch was tested with example presented in this commit message.
>>
>> [1]http://ftp.us.debian.org/debian/pool/main/q/qemu/
>> [1]https://www.linaro.org/blog/emulate-windows-on-arm/
>>
>> Signed-off-by: Pierrick Bouvier<pierrick.bouvier@linaro.org>
>> ---
>>    linux-user/syscall.c | 20 ++++++++++++--------
>>    1 file changed, 12 insertions(+), 8 deletions(-)
>>
>> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
>> index 08162cc966..90777c5833 100644
>> --- a/linux-user/syscall.c
>> +++ b/linux-user/syscall.c
>> @@ -659,6 +659,7 @@ safe_syscall4(pid_t, wait4, pid_t, pid, int *, status, int, options, \
>>    #endif
>>    safe_syscall5(int, waitid, idtype_t, idtype, id_t, id, siginfo_t *, infop, \
>>                  int, options, struct rusage *, rusage)
>> +safe_syscall3(int, execve, const char *, filename, char **, argv, char **, envp)
> 
> I guess we'll quickly get a build-time error if there's no host execve syscall
> (which looks to be, eventually, compat-only).
>

Out of curiosity, is there any plan for deprecation (or removal) of 
execve on a specific architecture? I thought it had to stay there, just 
for backward compatibility.

> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> 
> 
> r~