Re: [PATCH] linux-user: Improve strace output for read() and getcwd()

[Aleksandar Markovic <aleksandar.m.mail@gmail.com>]

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On Sunday, November 24, 2019, Helge Deller <deller@gmx.de> wrote:

> On 24.11.19 13:10, Aleksandar Markovic wrote:
> >
> >
> > On Sunday, November 24, 2019, Helge Deller <deller@gmx.de <mailto:
> deller@gmx.de>> wrote:
> >
> >     On 23.11.19 12:34, Aleksandar Markovic wrote:
> >     > On Thursday, November 21, 2019, Helge Deller <deller@gmx.de
> <mailto:deller@gmx.de> <mailto:deller@gmx.de <mailto:deller@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...
> >
> >
> > ... but I cannot see passed arguments (for example, whether NULL is
> passed, or somrthing that looks like a real address)...
>
> Right.
> And you won't see that in native strace either...
> If you have an idea, just let me know.
>
>
Helge, I do have an idea how to amend your proposal that should give
us, believe it or not, "best of both worlds", but let's wait few days, I
don't have any dev setup ar hand at the moment to check it in action, and
on top of that I am swamped with other unrelated tasks. In the meantime,
maybe other guys will say something too, or perhaps you would have
something new as well.

Still, by no means, I see this as a (potential) *huge* improvement of QEMU
strace. Thanks!

Yours,
Aleksandar



> Helge
>
> >
> >
> >
> >     Helge
> >
> >     >
> >     > Yours,
> >     > Aleksandar
> >     >
> >     >
> >     >
> >     >
> >     >     Signed-off-by: Helge Deller <deller@gmx.de <mailto:
> deller@gmx.de> <mailto:deller@gmx.de <mailto:deller@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,
> >     >
> >
>
>

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