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Re: [PATCH] linux-user: Improve strace output for read() and getcwd()
From: |
Aleksandar Markovic |
Subject: |
Re: [PATCH] linux-user: Improve strace output for read() and getcwd() |
Date: |
Sun, 24 Nov 2019 14:31:57 +0100 |
On Sunday, November 24, 2019, Helge Deller <address@hidden> wrote:
On 24.11.19 13:10, Aleksandar Markovic wrote:
>
>
> On Sunday, November 24, 2019, Helge Deller <address@hidden <mailto:address@hidden>> wrote:
>
> On 23.11.19 12:34, Aleksandar Markovic wrote:
> > On Thursday, November 21, 2019, Helge Deller <address@hidden <mailto:address@hidden> <mailto:address@hidden <mailto:address@hidden>>> 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 <address@hidden <mailto:address@hidden> <mailto:address@hidden <mailto:address@hidden>>>
> >
> > 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,
> >
>
- [PATCH] linux-user: Improve strace output for read() and getcwd(), Helge Deller, 2019/11/21
- Re: [PATCH] linux-user: Improve strace output for read() and getcwd(), no-reply, 2019/11/21
- Re: [PATCH] linux-user: Improve strace output for read() and getcwd(), Aleksandar Markovic, 2019/11/23
- Re: [PATCH] linux-user: Improve strace output for read() and getcwd(), Helge Deller, 2019/11/24
- Re: [PATCH] linux-user: Improve strace output for read() and getcwd(), Aleksandar Markovic, 2019/11/24
- Re: [PATCH] linux-user: Improve strace output for read() and getcwd(), Helge Deller, 2019/11/24
- Re: [PATCH] linux-user: Improve strace output for read() and getcwd(),
Aleksandar Markovic <=
- Re: [PATCH] linux-user: Improve strace output for read() and getcwd(), Aleksandar Markovic, 2019/11/25
- Re: [PATCH] linux-user: Improve strace output for read() and getcwd(), Helge Deller, 2019/11/25
- Re: [PATCH] linux-user: Improve strace output for read() and getcwd(), Aleksandar Markovic, 2019/11/25
- Re: [PATCH] linux-user: Improve strace output for read() and getcwd(), Laurent Vivier, 2019/11/26