Re: port-amd64/51700: exect(NULL,NULL,NULL) generates 15859 times SIGTRAP on amd64

[Kamil Rytarowski <n54%gmx.com@localhost>]

The following reply was made to PR port-amd64/51700; it has been noted by GNATS.

From: Kamil Rytarowski <n54%gmx.com@localhost>
To: Paul Goyette <paul%whooppee.com@localhost>, gnats-bugs%NetBSD.org@localhost
Cc: 
Subject: Re: port-amd64/51700: exect(NULL,NULL,NULL) generates 15859 times
 SIGTRAP on amd64
Date: Fri, 9 Dec 2016 11:38:48 +0100

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 From: Kamil Rytarowski <n54%gmx.com@localhost>
 To: Paul Goyette <paul%whooppee.com@localhost>, gnats-bugs%NetBSD.org@localhost
 Message-ID: <d79c070c-cc13-e1f5-b599-e39a9fd38a81%gmx.com@localhost>
 Subject: Re: port-amd64/51700: exect(NULL,NULL,NULL) generates 15859 times
  SIGTRAP on amd64
 References: <pr-port-amd64-51700%gnats.netbsd.org@localhost>
  <20161209063718.626CA7A364%mollari.NetBSD.org@localhost>
  <20161209095501.35ABE7A330%mollari.NetBSD.org@localhost>
  <Pine.NEB.4.64.1612091800580.7853%speedy.whooppee.com@localhost>
 In-Reply-To: <Pine.NEB.4.64.1612091800580.7853%speedy.whooppee.com@localhost>
 
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 Good point, if exect(3) is going to be a programmer-friendly utility
 (and since it already reserves its entry in libc), I'm for a breakpoint
 on the main() function.
 
 In that scenario we would get debugger code like:
 
 int status;
 child =3D fork();
 if (child =3D=3D 0)
     exect("/bin/cat", "cat", NULL);
 wait(&status);
 ptrace(...)
 
 On 09.12.2016 11:03, Paul Goyette wrote:
 > Just curious....  (private response, feel free to forward if you find i=
 t
 > useful)
 >=20
 >> This sounds to me like exect(3) should be equivalent to PT_TRACE_ME &
 >> execve(2) & raise(SIGTRAP with si_code=3D3DTRAP_TRACE) on first instru=
 ction
 >=20
 > The term "first instruction" could be interpreted as the beginning of
 > the executable's main() procedure.  Or it could be interpreted as the
 > beginning of crt0 code for the executable.
 >=20
 > I'm not sure which one you mean, but whatever decision is made, it need=
 s
 > to be clearly and unambiguously documented.  :)
 >=20
 >=20
 >=20
 >=20
 > On Fri, 9 Dec 2016, Kamil Rytarowski wrote:
 >=20
 >> The following reply was made to PR port-amd64/51700; it has been noted=
 
 >> by GNATS.
 >>
 >> From: Kamil Rytarowski <n54%gmx.com@localhost>
 >> To: gnats-bugs%NetBSD.org@localhost
 >> Cc:
 >> Subject: Re: port-amd64/51700: exect(NULL,NULL,NULL) generates 15859
 >> times
 >> SIGTRAP on amd64
 >> Date: Fri, 9 Dec 2016 10:47:52 +0100
 >>
 >> This is an OpenPGP/MIME signed message (RFC 4880 and 3156)
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 >> From: Kamil Rytarowski <n54%gmx.com@localhost>
 >> To: gnats-bugs%NetBSD.org@localhost
 >> Message-ID: <2044468e-027f-b38c-b17e-d9a23c55b820%gmx.com@localhost>
 >> Subject: Re: port-amd64/51700: exect(NULL,NULL,NULL) generates 15859
 >> times
 >>  SIGTRAP on amd64
 >> References: <pr-port-amd64-51700%gnats.netbsd.org@localhost>
 >>  <20161209063718.626CA7A364%mollari.NetBSD.org@localhost>
 >>  <20161209064000.613C97A364%mollari.NetBSD.org@localhost>
 >> In-Reply-To: <20161209064000.613C97A364%mollari.NetBSD.org@localhost>
 >>
 >> --kwFMJQnhwOSeUsptJJ1MMGmLvXhLtXrTK
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 >>
 >> Since this interface is apparently unclear I will add few sentences.
 >>
 >> The exect(3) call is an extension to execve(2),  it's added
 >> functionality is to raise SIGTRAP with si_code TRAP_TRACE.
 >>
 >> Currently mainly vax, i386 and amd64 implement this interface through
 >> libc, however with this is more a hack, as these port set machine flag=
 
 >> used to step the code and resumes with execve(2).
 >>
 >> If we want to keep its implementation in libc I think the best solutio=
 n
 >> is to switch this code to the sequence of raise(3) && execve(2) for al=
 l
 >> ports (how to set si_code to TRAP_TRACE in this case?).
 >>
 >> However I think it's little point to trace libc's internals and it mig=
 ht
 >> be more useful to raise signal just before executing the first
 >> instruction and this is doable
 >>
 >> The exect(3) function was added on day0 (inherited from 386bsd) and
 >> perhaps unused since then. The man-page describes it as follows (text
 >> unmodified since that time):
 >>
 >>   The function exect() executes a file with the program tracing
 >>   facilities enabled (see ptrace(2)).
 >>
 >> This sounds to me like exect(3) should be equivalent to PT_TRACE_ME &
 >> execve(2) & raise(SIGTRAP with si_code=3D3DTRAP_TRACE) on first
 >> instruction=3D
 >>
 >> of the new image.
 >>
 >> This call is currently unimplemented port-wide and perhaps not really
 >> functional with ports that implement it either, if we want to fix it I=
 
 >> think we are free to alter its meaning and make it more useful now.
 >>
 >> I checked 386bsd sources and it's the same as the current i386 version=
 
 >> -- without particular users. I think the proposed scenario with
 >> PT_TRACE_ME is the only variation that might be useful.
 >>
 >> exect(3) is kept in FreeBSD with the original questionable behavior an=
 d
 >> absent in OpenBSD
 >>
 >>
 >> --kwFMJQnhwOSeUsptJJ1MMGmLvXhLtXrTK--
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 >=20
 > +------------------+--------------------------+------------------------=
 +
 > | Paul Goyette     | PGP Key fingerprint:     | E-mail addresses:      =
 |
 > | (Retired)        | FA29 0E3B 35AF E8AE 6651 | paul at whooppee.com   =
 |
 > | Kernel Developer | 0786 F758 55DE 53BA 7731 | pgoyette at netbsd.org =
 |
 > +------------------+--------------------------+------------------------=
 +
 
 
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