record: Fix crash when the root is not existed

[zyxeeker]

This patch to fix crash when the root is not existed, due to some embed devices maybe not have it.
source:

static void unlink_shmem_list(void)
{
        ...
	list_for_each_entry_safe(sl, tmp, &shmem_need_unlink, list) {
		...
		struct dirent **shmem_bufs;
		sscanf(sl->id, "/uftrace-%[^-]-%*d-%*d", shmem_session);
		...
		free(shmem_bufs);
                ...
	}
}

My SOC do not have /dev/shm/ and shmem_bufs's value has not initialized, so scandir() not flush shmem_bufs and it has bad address, then bad thing happened:

/userdata # ./uftrace record -vv ./prog_test
...
uftrace: flushing /uftrace-bd6de237f8a0b25b-1383-000
uftrace: unlink for session: bd6de237f8a0b25b
munmap_chunk(): invalid pointer
Aborted (core dumped)

So I add access() to fix it:

static void unlink_shmem_list(void)
{
        ...
	list_for_each_entry_safe(sl, tmp, &shmem_need_unlink, list) {
		...
		/* check the root is existed (due to some embed devices maybe not have it) */
		if (access(uftrace_shmem_root(), F_OK) == 0) {
			num = scandir(uftrace_shmem_root(), &shmem_bufs, filter_shmem, alphasort);
			for (i = 0; i < num; i++) {
				...
			}

			free(shmem_bufs);
		}
		else {
			pr_dbg2("access %s failed, err: %s\n", uftrace_shmem_root(), strerror(errno));
		}
                ...
	}
}

[namhyung]

Thanks for your fix. It looks good but we have 3 calls to uftrace_shmem_root() now. I think it's better to add a local variable for it.

[namhyung]

Also I'm curious if your system doesn't have a temp directory at all.

[zyxeeker]

Thanks your reply!

Thanks for your fix. It looks good but we have 3 calls to uftrace_shmem_root() now. I think it's better to add a local variable for it.

The suggestion is good and I will check it again, so I will convert the issue to draft.

Also I'm curious if your system doesn't have a temp directory at all.

I have it, but many embed devices will use static /dev when RAM is limited, so maybe not have shm.
I will change pr_dbg2 to pr_warn to remind others.

[namhyung]

I think you can saved the result of uftrace_shmem_root() instead.

But I'm curious if it works on a system without shmem..

[zyxeeker]

I think you can saved the result of uftrace_shmem_root() instead.

Thanks, I will modify it

But I'm curious if it works on a system without shmem..

Sorry for my bad explain, I mean mostly embedded devices will use static /dev and maybe not have shm folder.
Although create shm folder will solve this problem, but I think that`s not a best way.