On 23.11.19 12:34, Aleksandar Markovic wrote:
> On Thursday, November 21, 2019, Helge Deller <del...@gmx.de
> <mailto:del...@gmx.de>> wrote:
>
> The strace functionality in qemu-user lacks the possibility to trace
> which real values get returned to pointers in userspace by syscalls.
>
> For example, the read() and getcwd() syscalls currently only show the
> destination address where the syscalls should put the return values:
> 2532 read(3,0xff80038c,512) = 512
> 2532 getcwd(0x18180,4096) = 9
>
> With the patch below, one now can specify in print_syscall_late() which
> syscalls should be executed first, before they get printed.
> After adding the read() and getcwd() syscalls, we now get this output in
> with strace instead:
> 1708
> read(3,"\177ELF\1\2\1\3\0\0\0\0\0\0\0\0\0\3\0\17\0\0\0\1\0\2bl\0\0\04"...,512)
> = 512
> 1708 getcwd("/usr/bin",4096) = 9
>
> This patch adds just the framework with the respective implemenations for
> read() and getcwd(). If applied, more functions can be added easily later.
>
>
> Great out-of-the-box idea! However, there are some things that are
> not thought over yet. There is a good reason why strace happens
> "early": if something crash-like happens during syscall translation,
> we still have trace of original target syscall. In case of "late"
> flavor, we don't have anything. Another potentially problematic
> aspect is that end user certainly should know whether the trace was
> done "early" or "late" - otherwise, there will be, for certain, cases
> of misinterpretation / misleading / confusion of end users.
Thanks for the feedback - I'm shortly sending a v2 version which
prints the syscall name with an open parenthesis, e.g. "read(" or "getcwd(",
before calling the syscall. That way you can see where it crashed/hangs...
Helge
>
> Yours,
> Aleksandar
>
>
>
>
> Signed-off-by: Helge Deller <del...@gmx.de <mailto:del...@gmx.de>>
>
> diff --git a/linux-user/strace.c b/linux-user/strace.c
> index de43238fa4..ff254732af 100644
> --- a/linux-user/strace.c
> +++ b/linux-user/strace.c
> @@ -61,6 +61,7 @@ UNUSED static void print_open_flags(abi_long, int);
> UNUSED static void print_syscall_prologue(const struct syscallname *);
> UNUSED static void print_syscall_epilogue(const struct syscallname *);
> UNUSED static void print_string(abi_long, int);
> +UNUSED static void print_encoded_string(abi_long addr, unsigned int
> maxlen, int last);
> UNUSED static void print_buf(abi_long addr, abi_long len, int last);
> UNUSED static void print_raw_param(const char *, abi_long, int);
> UNUSED static void print_timeval(abi_ulong, int);
> @@ -1204,6 +1205,37 @@ print_syscall_epilogue(const struct syscallname
> *sc)
> gemu_log(")");
> }
>
> +#define MAX_ENCODED_CHARS 32
> +static void
> +print_encoded_string(abi_long addr, unsigned int maxlen, int last)
> +{
> + unsigned int maxout;
> + char *s, *str;
> +
> + s = lock_user_string(addr);
> + if (s == NULL) {
> + /* can't get string out of it, so print it as pointer */
> + print_pointer(addr, last);
> + return;
> + }
> +
> + str = s;
> + gemu_log("\"");
> + maxout = MIN(maxlen, MAX_ENCODED_CHARS);
> + while (maxout--) {
> + unsigned char c = *str++;
> + if (isprint(c)) {
> + gemu_log("%c", c);
> + } else {
> + gemu_log("\\%o", (unsigned int) c);
> + }
> + }
> + unlock_user(s, addr, 0);
> +
> + gemu_log("\"%s%s", maxlen > MAX_ENCODED_CHARS ? "..." : "",
> + get_comma(last));
> +}
> +
> static void
> print_string(abi_long addr, int last)
> {
> @@ -1633,6 +1665,19 @@ print_futimesat(const struct syscallname *name,
> }
> #endif
>
> +#ifdef TARGET_NR_getcwd
> +static void
> +print_getcwd(const struct syscallname *name,
> + abi_long arg0, abi_long arg1, abi_long arg2,
> + abi_long arg3, abi_long arg4, abi_long arg5)
> +{
> + print_syscall_prologue(name);
> + print_string(arg0, 0);
> + print_raw_param("%u", arg1, 1);
> + print_syscall_epilogue(name);
> +}
> +#endif
> +
> #ifdef TARGET_NR_settimeofday
> static void
> print_settimeofday(const struct syscallname *name,
> @@ -2428,6 +2473,20 @@ print_fstatat64(const struct syscallname *name,
> #define print_newfstatat print_fstatat64
> #endif
>
> +#ifdef TARGET_NR_read
> +static void
> +print_read(const struct syscallname *name,
> + abi_long arg0, abi_long arg1, abi_long arg2,
> + abi_long arg3, abi_long arg4, abi_long arg5)
> +{
> + print_syscall_prologue(name);
> + print_raw_param("%d", arg0, 0);
> + print_encoded_string(arg1, arg2, 0);
> + print_raw_param("%u", arg2, 1);
> + print_syscall_epilogue(name);
> +}
> +#endif
> +
> #ifdef TARGET_NR_readlink
> static void
> print_readlink(const struct syscallname *name,
> diff --git a/linux-user/strace.list b/linux-user/strace.list
> index d49a1e92a8..220b1f4c46 100644
> --- a/linux-user/strace.list
> +++ b/linux-user/strace.list
> @@ -272,7 +272,7 @@
> { TARGET_NR_getcpu, "getcpu" , "%s(%p,%d)", NULL, NULL },
> #endif
> #ifdef TARGET_NR_getcwd
> -{ TARGET_NR_getcwd, "getcwd" , "%s(%p,%d)", NULL, NULL },
> +{ TARGET_NR_getcwd, "getcwd" , NULL, print_getcwd, NULL },
> #endif
> #ifdef TARGET_NR_getdents
> { TARGET_NR_getdents, "getdents" , NULL, NULL, NULL },
> @@ -1080,7 +1080,7 @@
> { TARGET_NR_quotactl, "quotactl" , NULL, NULL, NULL },
> #endif
> #ifdef TARGET_NR_read
> -{ TARGET_NR_read, "read" , "%s(%d,%#x,%d)", NULL, NULL },
> +{ TARGET_NR_read, "read" , NULL, print_read, NULL },
> #endif
> #ifdef TARGET_NR_readahead
> { TARGET_NR_readahead, "readahead" , NULL, NULL, NULL },
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index ce399a55f0..c0079ca2b7 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -12069,6 +12069,21 @@ static abi_long do_syscall1(void *cpu_env, int
> num, abi_long arg1,
> return ret;
> }
>
> +/*
> + * True if this syscall should be printed after having called the native
> + * syscall, so that values which are fed back to userspace gets printed.
> + */
> +static int print_syscall_late(int syscall)
> +{
> + switch (syscall) {
> + case TARGET_NR_getcwd:
> + case TARGET_NR_read:
> + return 1;
> + default:
> + return 0;
> + }
> +}
> +
> abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
> abi_long arg2, abi_long arg3, abi_long arg4,
> abi_long arg5, abi_long arg6, abi_long arg7,
> @@ -12095,9 +12110,16 @@ abi_long do_syscall(void *cpu_env, int num,
> abi_long arg1,
> arg2, arg3, arg4, arg5, arg6, arg7, arg8);
>
> if (unlikely(do_strace)) {
> - print_syscall(num, arg1, arg2, arg3, arg4, arg5, arg6);
> + int late_printing;
> + late_printing = print_syscall_late(num);
> + if (!late_printing) {
> + print_syscall(num, arg1, arg2, arg3, arg4, arg5, arg6);
> + }
> ret = do_syscall1(cpu_env, num, arg1, arg2, arg3, arg4,
> arg5, arg6, arg7, arg8);
> + if (late_printing) {
> + print_syscall(num, arg1, arg2, arg3, arg4, arg5, arg6);
> + }
> print_syscall_ret(num, ret);
> } else {
> ret = do_syscall1(cpu_env, num, arg1, arg2, arg3, arg4,
>
